From the second edition (1989):
(kənˈspɪrəsɪ) [f. L. conspīrātio conspiration, with substitution of the ending -acy q.v. A single example of conspiratie in 16th c. F. is given by Godefroy.]

1. a. The action of conspiring; combination of persons for an evil or unlawful purpose.

c1386 Chaucer Monk's T. 621 Brutus and Cassius‥Ful prively hath made conspiracie Agains this Julius in subtil wise. 1389 in Eng. Gilds (1870) 5 Enpresoned falslich‥by fals conspiracie. 1602 Marston Antonio's Rev. v. i, Made a partner in conspiracie. 1610 Shakes. Temp. ii. i. 301 Open-ey'd Conspiracie His time doth take. 1667 Milton P.L. ii. 751 Combin'd In bold conspiracy against Heav'ns King. a1832 Bentham Just. & Codif. Petit. Wks. 1843 V. 485 In the very import of the word conspiracy is therefore included the conspiracy to do a bad thing. 1841–4 Emerson Ess., Self-Reliance Wks. (Bohn) I. 20 Society everywhere is in conspiracy against the manhood of every one of its members.

b. Law.

1863 H. Cox Instit. i. xi. 275 The crime of conspiracy consists in the agreement of two or more persons to do an illegal act, or to do a lawful act by unlawful means.

2. a. (with a and pl.) A combination of persons for an evil or unlawful purpose; an agreement between two or more persons to do something criminal, illegal, or reprehensible (especially in relation to treason, sedition, or murder); a plot. Also in phr. conspiracy of silence.

c1386 Chaucer Doctor's T. 149 Whan schapen was al this conspiracye Fro poynt to poynt. 1494 Fabyan Chron. v. lxxxvi. 64 Hauynge knowlege of the sayde conspiracy. 1553 Eden Treat. Newe Ind. (Arb.) 34 Fearing greater deceytes and conspiracies. 1632 Star Chamb. Cases (Camden) 170, I hould here is a conspiracie by Travers and Frost and his daughter. 1647 Clarendon Hist. Reb. ii. (1843) 66/2 In all conspiracies there must be great secrecy. 1781 Gibbon Decl. & F. II. 81 A secret conspiracy had been formed against his person and government. 1839 James Louis XIV, I. 105 Rumours of a conspiracy became general. 1865 Mill Comte 199 M. Comte used to reproach his early English admirers with maintaining the ‘conspiracy of silence’ concerning his later performances. 1870 Gentl. Mag. V. 331 Compel them, if they must divert themselves with a species of amateur conspiracy, to enter into a conspiracy of silence. 1885 Church Reformer July 146/1 When such work as the Pall Mall has done is treated by the ordinary press with a conspiracy of silence, then it becomes our duty‥to thank that paper for its outspoken boldness. 1931 V. J. McGill Schopenhauer ix. 303 Their ‘conspiracy of silence’ had at last proved ineffective. 1944 G. B. Shaw Everybody's Pol. What's What? xxiv. 223 The conspiracy of silence about the blunders and failures of medicine. 1965 Times Lit. Suppl. 27 May 437/3 It semed perfectly correct to me when I read the article in question, although I find that English grammars seem to maintain a conspiracy of silence on the subject.

b. A body or band of conspirators. Obs.

1555 Eden Decades 53 The captayne of this conspiracie was slayne. 1571 Golding Calvin on Ps. lxiv. 2 Conspiracie‥may bee taken as well for a companye that consult about mischeef, as for the mischeef itself they have devysed. 1600 Holland Livy vii. xli. 279 Urged by those of the conspiracie [ab conjuratis].

3. fig. Union or combination (of persons or things) for one end or purpose; harmonious action or effort; = conspiration 3. (In a good or neutral sense.) Obs. or arch.

1538 Starkey England i. i. 11 That thys cyvyle lyfe was‥a conspyracy in honesty and vertue. 1580 Sidney Arcadia iii. 382 (D.) So is the conspiracy of her several graces, held best together to make one perfect figure of beauty. a1677 Barrow Wks. (1830) I. 191 There will be a conspiracy and faithful correspondence between our mind and our tongue. 1691–8 Norris Pract. Disc. 229 To discern this Harmony and beautiful conspiracy of things. 1847 Emerson Ode to Beauty Wks. (Bohn) I. 450 All that's good and great with thee Works in close conspiracy.