From the second edition (1989):
British, a. (n.)
(ˈbrɪtɪʃ) Forms: 1 Brettisc, Bryttisc, Brittisc (Brytisc), 4 Bruttische, 5 Brytysshe, 6 Brutish, 7 Brittish, Britysh, 6– British. [OE. Brettisc, etc., f. Bret, pl. Brett-as, Bryttas, Brittas, the natives of ancient Britain, the Britons: see brit and -ish. The modern spelling is influenced by Latin.]

1. a. Of or pertaining to the ancient Britons. Now chiefly in ethnological and archæological use.

a855 O.E. Chron. an. 508 Her Cerdic and Cynric ofsloon ænne Brettisc [Laud MS. Bryttiscne] cyning. a1000 Ibid. (Laud) Introd., Her sind on þis ilande fif eþeode · Englisc and Brittisc and Wilsc, and Scyttisc and Pyhtisc and Boc Leden. a1100 Ibid. an. 1075 Se ylca Raulf wæs Bryttisc on his moder healfe, and his fæder wæs Englisc. 1605 Shakes. Lear iii. iv. 189 Fie, foh, and fumme, I smell the blood of a Brittish man. c1645 Howell Lett. (1650) I. 377 He calls‥Helen an English woman; whereas, she was purely British, and that there was no such nation upon earth called English at that time. 1780 Cowper Boadicea i, The British warrior queen, Bleeding from the Roman rods. 1870 Knight Hist. Eng. i. 3 A road, acknowledged to be British, still crosses Salisbury Plain.

b. Of or pertaining to the Celtic (Brythonic) language of the ancient Britons; later, = Welsh, occas. Cornish. Also as n.

c1205 Layaman's Brut (1847) (E.E.T.S.) 13393 Nu ic þe wulle teche. Bruttisce spæche. 1387 Trevisa Higden (Rolls) II. 79 Þis citee‥in Brittische speche heet Caerleon,‥and Chestren in Englisshe. 1550 W. Salesbury Brief & Plain Introd. Welsh sig. A. i, A briefe and a playne introduction, teachyng how to pronounce the letters in the British tong, (now commenly called Walsh). Ibid. sig. B. i, In Britishe or Walshe in euery worde hath the true pronunciation of A in latine. 1637 Holland tr. Camden's Britannia 472 It coupleth it selfe with the Yare, which the Britans called Guerne‥of Aldertrees, no doubt, so termed in British wherewith its overshadowed. 1662 Act of Uniformity 13–14 Chas. II, iv. §27 That the Book [of Common Prayer] hereunto annexed be truly and exactly translated into the British or Welsh Tongue. 1712 Life St. Winefride 61 Wen in the Old British Tongue signifies White. 1838 W. Howitt Rural Life Eng. II. xvi. 380 The British tongue here [in Tintagel] lingered till lately. 1962 G. Grigson Shell Country Bk. i. 112 Knowing no British, they would take this word avon for a name. 1972 W. B. Lockwood Panorama of Indo-European Langs. vi. 67 It is plain that the Celtic spoken in Britain was a language of the same general type as Gaulish; the two were closely related languages…We may call it British…It is often useful to be able to refer to Gaulish and British together; we may then speak of Gallo-Brittonic.

2. a. Of or belonging to Great Britain, or its inhabitants. In the earlier instances only a geographical term adopted from Latin; from the time of Henry VIII frequently used to include English and Scotch; in general use in this sense from the accession of James I, and in 17th c., often opposed to Irish; legally adopted at the Union in 1707. Now chiefly used in political or imperial connexion, as the British army, British colonies, British India, etc., British ambassador, consul, residents, etc.; also in scientific and commercial use, as British plants, British butterflies, British spirits.

1387 Trevisa Higden (1865) I. 271 Gallia‥is i-closed aboute‥wiþ þe Bruttische occean in þe west side. 1398 —— Barth. De P.R. xv. lxvi. (1495) 512 Fraunce‥endyth in the north at Brytysshe Occean. 1570–87 Holinshed Scot. Chron. (1806) I. 43 Amongst the Irish Scotishmen‥the petition of the British Scots. 1604 J. Dee in Hearne Coll. (1885) I. 64 This Britysh Empire. 1643 Script. Reas. for Defens. Armes 76 The extirpation of the Brittish Nation, and Protestant Religion in that kingdome [Ireland]. 1699 Garth Dispens. i. 7 How have I kept the Brittish Fleet at ease? 1706–7 Act of Union 6 Anne xi. §1 art. 8 Without any mixture of British or Irish salt. 1769 Burke Pres. St. Nat. Wks. II. 187 Every British merchant in Petersburgh. 1841 W. Spalding Italy & It. Isl. II. 393 His strange discussions on the British constitution. 1855 Tennyson Maud i. xiii. ii, A stony British stare. 1882 Garden 18 Feb. 112/1 Our common British Ivy.

b. British Empire: the empire consisting of Great Britain and the other British possessions, dominions, and dependencies; now replaced by the British Commonwealth (see commonwealth 4c). Cf. empire n. 5b.

1604 J. Dee Pet. to King, God‥make your Maiestie to be the most blessed and Triumphant Monarch, that euer this Brytish Empire, enioyed. 1768 Goldsmith (title) The present state of the British Empire in Europe, America, Africa and Asia, containing a concise account of our possessions in every part of the globe, [etc.]. 1813 Hector Campbell (title) The Impending Ruin of the British Empire, its Cause and Remedy considered. 1884, etc. [see commonwealth 4c]. 1887 [see empire n. 5a]. 1902 Encycl. Brit. XXXIII. 393/1 The British Empire League and the Imperial Trade Defence League endeavour to promote inter-Imperial trade.

c. British Isles: a geographical term for the islands comprising Great Britain and Ireland with all their offshore islands including the Isle of Man and the Channel Islands (see isle n. 1).

1621 Heylin Microcosmus 243 (page-heading) The Brittish Isles. 1792 A. Young Trav. France ii. 343 A territory, naturally so inconsiderable as the British isles, on comparison with France. 1888 A. J. Jukes-Browne Building of Brit. Isles 1 There have been many different arrangements of land and sea over the area where the British Isles now stand. 1916 G. B. Shaw Androcles & Lion Pref. p. li, Practically all the white inhabitants of the British Isles and the North American continent. 1960 C. Day Lewis Buried Day ii. 32 He was for ever buying, selling or exchanging books, many of them worthless, with correspondents all over the British Isles.

d. British English: the English language as spoken or written in the British Isles; esp. the forms of English usual in Great Britain, as contrasted with those characteristic of the U.S.A. or other English-speaking countries.

[1869 Ellis E.E. Pronunc. I. 20 Practically the speech of the American English is archaic with respect to that of the British English.] 1892 H. Sweet New Eng. Gram. i. 224 The influence of the vulgar London or ‘Cockney’ dialect is stronger in Australasian than in British English. 1926 H. W. Fowler Mod. Eng. Usage 52/2 These words do not mean in American‥use what they mean in British English. 1932 R. W. Chapman ‘Oxford’ English 540 This expression may be current in America, but it is not British English. 1967 ‘J. Cross’ To Hell for Half-a-crown x. 130 The English that Neumann was babbling was not‥British English‥but American.

3. Of or belonging to Brittany, Breton. Obs.

1602 Carew Cornwall 131b, One of their auncestours‥entertained a British Miller, as that people, for such idle occupations, proue more hardie then our owne.

4. ellipt. as n. pl. British people, soldiers, etc.

1641 in Miss Hickson Irel. 17th C. (1884) II. App. U. 363 [In county Monaghan] there being a little plantation of British, the rebels plundered the town. 1652 Ibid. (1884) I. xxxix. 245 As the Irish rebels marched through the said parish they murdered all the British they could lay their hands on. 1708 Lond. Gaz. No. 4459/3 The British had not a Man kill'd or wounded. 1844 H. H. Wilson Brit. India ii. vii. II. 269 Appearances began to assume an aspect most unfavourable to the British.

5. Comb., as British-born, British-built, British-owned adjs., British-man; British crown, a gold coin current in the reign of Charles I.; British disease, loosely applied to any fault or disorder considered typical of the British as a nation, esp. proneness to industrial unrest; cf. English disease (c) s.v. English a. 2e; British gum, a commercial name of dextrin; British-Israelite = Anglo-Israelite; so British-Israel (attrib.), British-Israelism; British Restaurant, a government-subsidized restaurant opened during the war of 1939–45; British school, a public elementary school, on the non-denominational or unsectarian basis of the ‘British and Foreign School Society’; British Standard Time, the time system introduced in Britain on 18 Feb. 1968, which is the same as Central European Time and is equivalent to the extension of Summer Time throughout the year; British Summer Time = summer-time 2; British thermal unit (see thermal a. 2); British warm (see warm n.2 2).

1796 Morse Amer. Geog. II. 108 Numbers of *British-born subjects. 1756 Act 29 Geo. II, xxxiv. §18 *British built Ships or Vessels. 1866 Crump Banking x. 224 Charles I—Gold [coins]—Three-pound piece, angel, unite, double-crown, *British-crown. 1971 Guardian 6 Mar. 11/5 The disruption is caused, in Ford's eyes, by the ‘*British Disease’—constant, unpredictable strikes. 1979 Dædalus Winter 14 The British, after years of vivisecting their malaise (the ‘British disease’), see a brighter future thanks to the combination of luck—the North Sea oil—and‥rediscovering self-restraint and incomes discipline. 1984 Financial Times 4 Feb. 4 The British disease in this context‥is high levels of overtime, much higher than on the continent. 1860 Mayne Exp. Lex., *British Gum (Chem.), term for a species of gum into which starch is converted when exposed to a temperature between 600° and 700°‥used as a substitute for gum Arabic in calico printing and other processes. 1907 (title) The *British-Israel Ecclesia. 1920 S. C. (title) *British-Israelism. 1934 Times Lit. Suppl. 1 Feb. 77/2 Are all the present millions of *British-Israelites mad? 1948 ‘N. Shute’ No Highway v. 133 He has been in trouble with the police arising out of his activities with the British Israelites. 1711 Shaftesbury Charac. (1737) III. 144 Had it happen'd to one of us *British-men to have been born at sea, cou'd we not therefore properly be call'd British-men? 1858 Merc. Mar. Mag. V. 308 *British-owned‥vessels. 1941 W. S. Churchill Minute 21 Mar. in Grand Alliance (1950) 663, I hope the term ‘Communal Feeding Centres’ is not going to be adopted.‥ I suggest you call them *‘British Restaurants’. 1950 A. Wilson Such Darling Dodos 152 Returning from lunch at the British Restaurant. 1967 Times 24 Oct. 11/3 The Secretary of State for Home Affairs announced on June 23 that a bill will be introduced early next session to extend ‘summer time’ permanently throughout the year.‥ We understand that the name *British Standard Time is being considered; this is ambiguous, is likely to lead to confusion and misunderstanding and should be rejected. 1970 A. P. Herbert In Dark 11 By the beginning of February some real advantages will at last be seen to flow from British Standard Time. 1930 *British Summer Time [see B.S.T. s.v. B III]. 1958 L. R. Muirhead Blue Guide to Northern Spain (ed. 2) p. cxxxvi, Railway time is now 1 hour later than Greenwich time, and coincides with British Summer Time when the latter is in force. 1970 A. P. Herbert In Dark 90 We arrived punctually at one o'clock Local Time, two o'clock Newfoundland Government Time, 1730 British Summer Time and 1530 Greenwich Mean Time.

6. the best of British luck, an expression of encouragement, often with the ironical implication that good luck will not be forthcoming. Also ellipt. best of British.

1961 C. Witting Driven to Kill i. 12 Here's my P.S.V. badge if you want to take the number—and the best of British luck to you. 1963 L. Meynell Virgin Luck ii. 39 As soon as they [sc. kittens and young birds] can fend for themselves they get shoved out into the world and the best of British luck to them. 1965 Eva-Lis Wuorio Z for Zaborra 190 The best of British luck to you. 1967 W. Keenan Lonely Beat vii. 74 ‘Best of British.’ With that the reporter vanished.

Hence British-hood, Britishness.

1883 A. Forbes in Ninet. Cent. Oct. 722 Their British-hood manifests itself in things big and in things little. 1872 S. Mostyn Perplexity III. iii. 46 His thorough Britishness.