From the second edition (1989):
unless, prep., conj., and n.
(ənˈlɛs) Forms: α. 5 of lasse, oo lesse, o less, oless(e, Sc. oles, 9 Sc. aless. β. 5 vpon less. γ. 5 in lasse, yn las, 5 in less (9 dial. inless), 6 inlesse. δ. 5 on lasse, 5–6 onlesse, 6 oon-, oneles, -lez, 5–7 onles, 9 dial. onless. ε. 5 vnlasse, 6– 7 vn-, unlesse, 6 vnles, 6– unless. [f. less a. 7, with the preps. of, in, upon, and on; the last of these by want of stress has been assimilated in form to the prefix un-1. Cf. less conj., also unleast, unlest.]

1. prep. phr. On a less or lower condition, requirement, footing, etc., than (what is specified).
With preceding negative, expressed or implied.

c1400 Mandeville (1919) xxi. 122 But þat may not be vpon less þan wee mowe falle toward heuene. c1400 [see less a. 7c]. 1475 Rolls of Parlt. VI. 127/1 His Land, which many persones‥fere to take‥, olesse then they myght be made verrey sure of payment. a1500 in C. Trice-Martin Chanc. Proc. 15th C. (1904) 5 Robert wil not suffre hym to be laten to baile on lasse than he will make‥a generall acquytaunce.

2. Except, if‥not:a. With retention of than (cf. prec.), but without a negative. Obs.

1431 Acts. Privy Counc. IV. 96 It shulde be entendede unto, namely, olesse þan before þt men can se‥þe meenes‥of ferþer conduyt of þe werre. c1449 Pecock Repr. iii. xvi. 386 Alle hise successouris ben‥excludid for euer, in lasse thanne the same good be ȝouun aȝen. 1467 in Eng. Gilds (1870) 408 Vppon peyn of euery man so failynge, vnlesse then he haue a sufficient depute, of xl.d. 1530 Tindale Gen., Prol. Use Script. Av, Inlesse then we entend to be ydle disputers, and braulers aboute vayne wordes.

b. Followed by that. Obs.

1440 in Wars Eng. in France (1864) II. 458 The kyng conceyueth wele that onlesse that it like him so to tendre the said duc he [etc.]. 1470–85 Malory Arthur i. x. 47 Onlesse that our kyng haue more chyualry,‥he shal be ouer~come. 1529 Wolsey in Cavendish Life (1825) II. 261 Onles that yow‥do helpe & releve me therin. 1534 in Leadam Star Chamber Cases (Selden) II. 211 [They] cowde not‥sell so myche‥onelez that they wold sell so reassonable a pennyworthe. 1596 Spenser F.Q. vi. iii. 39 Ne would I gladly combate with mine host,‥Vnlesse that I were thereunto enforst.

c. With omission of conjunction before the subordinate clause, and thus passing into conj. (Cf. 4.) Freq. in phr. unless and until.

1509 Fisher Serm. Wks. (1876) 277 There is no man‥oneles he haue it by reuelacyon that knoweth certaynly [etc.]. 1542 Lam. & Piteous Treat. in Harl. Misc. (1745) IV. 505/2 It was a verey daungerous and ieoperdous Thinge,‥inlesse they had been‥weryd by longe Soiourynge. 1563 Mirr. Mag., Blacksmith lxviii, For one [talent] is to much, onles it be well spent. 1607 in Eng. Gilds (1870) 442 Margory Davies‥wold not remove her habitacion onles she might haue a way‥to passe [etc.]. 1662 Stillingfl. Orig. Sacræ ii. iii. §5 Will God condemn them for that, which it was impossible they should have, unless God gave it them? 1710 Swift Jrnl. to Stella 8 Oct., I was at a loss to-day for a dinner, unless I would have gone a great way. 1752 Young Brothers i. i, Dominion, and the princess, both are lost, Unless you gain the king. 1820 Southey Wesley II. 211 No person was admitted to this rank, unless he were thought competent by the preachers of the circuit. 1877 Ruskin Fors Clav. lxxx. VII. 234, I am never angry with anybody unless they deserve it. 1937 D. Jones in Le Maître Phonétique Apr.–June (Suppl.), We should as a rule stick to that pronunciation unless and until we find another native whose speech we have reason to think is more characteristic. 1956 A. Wilson Anglo-Saxon Attitudes ii. ii. 335 Mother and son had both arrived with the fixed determination of not leaving unless and until either of the two women‥should have paid handsomely to secure their departure. 1983 Times 1 Feb. 15/1 Unless and until the government also proposes the abolition‥of the GLC, the savings in public money are likely to be minimal.

d. Followed by a prepositional or participial clause without verb, or by when, where, etc.

1548–9 (Mar.) Bk. Com. Prayer, Offices, Bapt. Priv. Ho., [That] the people‥defer not the Baptisme of infantes‥onlesse vp on a greate & reasonable cause. 1610 Fletcher Faithful Sheph. v. v, [Let] not wine, Unless in sacrifice, or rites divine, Be ever known of Shepherd. 1681 Dryden Abs. & Achit. i. 590 Nor ever was he known‥[to] Curse, unless against the Government. 1721 Bradley Philos. Acc. Wks. Nat. 77 We had no Frost or Snow‥, unless in the most Inland Parts. 1749 Fielding Tom Jones xvii. ix, Jones passed above twenty-four melancholy hours by himself, unless when relieved by the company of Partridge. 1789 C. Smith Ethelinde (1814) II. 147 But I dare not shew them, unless to you. 1818 Scott Rob Roy v, A beautiful horse, jet black, unless where he was flecked by spots of‥foam. 1897 M. Kingsley W. Africa 439 They never wear clothes unless compelled to.

3. prep. a. Except, but.

1531–2 Act 23 Hen. VIII, c. 1 That no suertye be taken onles suche as maye dyspende‥yerly‥xxvi.s. viii.d. 1563 Hill Art Garden. ii. lxiv. (1574) 132 The floures‥ought then to be gathered‥vnlesse the Lilly‥and Rose. 1600 Heywood If you know not me Wks. 1874 I. 205 All forbeare this place, vnlesse the Princess. 1683 D. A. Art Converse 117 They say nothing unless a meer yes sir or no Madam. 1709 T. Robinson Nat. Hist. Westmoreld. viii. 53 Inconsistent‥with the Nature of Lead, Copper, Coal, or any other Mineral, unless Iron. 1796 Morse Amer. Geog. II. 33 Unless the Swedish part,‥the Laplanders can be said to be under no regular government. 1886 Pall Mall. G. 4 Aug. 1/2 He did not believe that he would ever obtain anything‥unless a species of elevated poor-law system of government.

b. Without; but for. Obs.

1536 St. Papers Hen. VIII (1830) I. 469 Soo that, unles the most infinite mercy of God, both bodyes and soules shuld perishe togither. 1796 Mrs. J. West Gossip's Story I. 190 You instilled into my infant soul principles, which, unless my own fault, must insure my present and future happiness.

4. conj. a. Lest. Obs.

1508 Fisher 7 Penit. Ps. cii. Wks. (1876) 142, I fere‥oneles I shall fall agayne amonge those theues. 1543 Becon Invect. agst. Swearing Eiiijb, I feare vnlesse we shall be redy‥to runne hedlong into hell fyre. a1592 Greene Alphonsus i. i, Presume not, villaine, further for to go, Vnles you do at length the same repent.

b. But that. Obs.— 1

1608 in Harl. Misc. (1744) I. 181 A Flea shall not frisk forth, unless they comment upon her.

5. n. An utterance or instance of the word; a reservation, proviso.

1861 Dickens in Pall Mall G. 24 Sept. (1891) 3/2 Let us have no unlesses, sir. 1904 R. Hichens Woman with Fan vii, There's very often an unless hanging about, like a man at a street corner.