From the second edition (1989):
(ˈθjuːliː) Forms: 1 Thila, 1, 4–5 Tyle, Tile, 7 Thyle, Tule, 6– Thule. [L. Thūlē (Thȳle) = Gr. Θούλη (Θύλη), proper name of unknown origin.]

1. a. The ancient Greek and Latin name (first found in Polybius's account of the voyage of Pytheas) for a land six days' sail north of Britain, which he supposed to be the most northerly region in the world.
(Thule has been variously conjectured to be the Shetland Islands (so app. in Pliny and Tacitus), Iceland, the northern point of Denmark, or some point on the coast of Norway.)

c888 K. Ælfred Boeth. xxix. §3 Oð ðæt iland þe we hatað Tyle. c893 —— Oros. i. i. §27 Be westannorðan Ibernia is þæt ytemeste land þæt man hæt Thila. a1000 Boeth. Metr. xvi. 15 An ilond‥ is Tile haten. c1374 Chaucer Boeth. iii. met. v. (Camb. MS.), Þe last Ile in þe see þat hyhte tyle [v.r. tile]. 1387 Trevisa Higden (Rolls) I. 325 Tyle is sixe dayes seillynge oute of Bretayne. 1598 Sylvester Du Bartas ii. ii. iv. Columnes 230 From Africa to Thule's farthest Flood. 1613–16 W. Browne Brit. Past. i. v, Monster-breeding Nyle Or through the North to the unpeopled Thyle. 1665 Sir T. Herbert Trav. (1677) 2 The fortunate Islands‥about which has been no small difference amongst Writers. Some placing them at the Azores‥but the Commentator upon Horace near the Ultima Thule. a1688 J. Wallace (title) An Essay Concerning the Thule of the Ancients. 1730–46 Thomson Autumn 864 Where the Northern Ocean‥Boils round the naked melancholy isles Of farthest Thule. 1847 Macaulay in Trevelyan Life (1876) II. 190 Where more than Thule's winter barbs the breeze.

b. transf. As the type of the extreme limit of travel and discovery, chiefly (after Latin usage) in the phrase ultima Thule (farthest Thule); hence fig. the highest or uttermost point or degree attained or attainable, the acme, limit; the lowest limit, the nadir.

1771 Smollett Humph. Cl. 3 Sept., I am now little short of the Ultima Thule, if this appellation properly belongs to the Orkneys or Hebrides. 1784–5 Ann. Reg. ii. 12/1 An unknown coast, which he [Cook] named Sandwich Land, the thule of the Southern hemisphere. 1828 Lights & Shades II. 136 The caricature of a fop, the ultima Thule of extravagant frippery. 1878 Times 10 May (Stanf.), The expedition reached their Ultima Thule. 1954 M. Lowry Let. 22 May (1967) 370 Before you write off that behaviour as being the ultima thule of ingratitude‥try to understand the effect your news‥had on me. 1976 L. Davidoff et al. in Mitchell & Oakley Rights & Wrongs of Women iv. 157 The one who had ‘fallen’ out of the respectable society‥to the ultima Thule of prostitution.

2. Archæol. (with pronunc. (θuːl, θjuːl)). Used chiefly attrib. to designate a prehistoric Eskimo culture widely distributed from Alaska to Greenland c 500–1400 a.d. [From the name Thule (now Dundas), a settlement in N.W. Greenland.]

1927 T. Mathiassen Archæol. of Central Eskimos II. i. 2 We have‥found remnants of an older culture which, after a locality outside the Central Eskimo territory, in North Greenland, we have called the Thule culture. 1935 Nature 3 Aug. 188/1 He [sc. Mathiassen] regards the Thule culture as originating in Asia. 1956 G. Freeman tr. Malaurie's Last Kings of Thule i. vii. 87 The Thule Culture—neo-Eskimo—would be derived from an anterior continental culture—palaeo-Eskimo. 1962 Times 4 Aug. 7/7 Within recent historical times Greenland has known three Eskimo cultures. The most recent is that of the Thule people who moved across Canada from Alaska. 1972 Country Life 12 Oct. 880 About ad 1100 a second Eskimo culture, known to archaeologists as the Thule period because it was first identified at Thule in Greenland, spread eastward from Alaska. Ibid. 881 The Thule Eskimos lived in stone houses. 1974 Encycl. Brit. Macropædia I. 1130/2 The spread of Thule‥has been traced eastward from Alaska, arriving in Greenland about 1200.‥ Later there was a resurge of Thule back toward the west, reaching all the way to Bering Strait. 1977 G. Clark World Prehist. (ed. 3) ix. 411 The Neo-Eskimo bearers of the Thule culture, immediate forebears of the existing population.