From the second edition (1989):
zany, n. (a.)
(ˈzeɪnɪ) Also 6–7 zani, zanie, 7 zane, zanee, zanni, (pl. zaneese), 8 zaney. [a. F. zani, or its source It. zani, zanni name of servants who act as clowns in the ‘Commedia dell' arte’.
Properly, the Venetian and Lombardic form of Gianni = Giovanni John (cf. Zanipolo the title of the church of St. John and St. Paul in Venice) used as an appellative for a porter (or the like) from the mountain country of Bergamo who had taken service in a seaside town.

In the following the French word is app. intended:—a1566 R. Edwards Damon and Pithias (1571) Fij, Iacke. Iebit avow mon companion. Grimme. Ihar vow pleadge pety Zawne. Iacke. Can you speake Frenche, here is a trimme colier by this day.]

1. A comic performer attending on a clown, acrobat, or mountebank, who imitates his master's acts in a ludicrously awkward way; a clown's or mountebank's assistant, a merry-andrew, jack-pudding; sometimes used vaguely for a professional jester or buffoon in general. Now Hist. or arch.

1588 Shakes. L.L.L. v. ii. 463 Some carry-tale, some please~man, some slight Zanie,‥That‥knowes the trick To make my Lady laugh. 1596 Lodge Wits Mis. Mivb, Here marcheth forth Scurilitie,‥the first time he lookt out of Italy into England, it was in the habite of a Zani. 1599 B. Jonson Ev. Man out of Hum. iv. i, Hee's like a Zani to a Tumbler, That tries trickes after him to make men laugh. 1601 Shakes. Twel. N. i. v. 96, I protest I take these Wisemen‥no better then the fooles Zanies. 1648 Winyard Mids.-Moon 2 Cheynell among the visitors, is a mountebanke extraordinary with 4 zanyes. 1652 Urquhart Jewel 104 They go‥in the disguise of a Zanni or Pantaloon to ventilate their fopperies. 1682 New News fr. Tory-Land 7 He may serve for some Zany to a Mountebank, to jest off Medicines for the Tooth~ach to the Rabble in Southwark. 1760 C. Johnston Chrysal lxix, A mountebank-doctor, and his zany. 1810 Crabbe Borough vii. 66 There was a time, when we beheld the quack, On public stage, the licensed trade attack; He made his labour'd speech with poor parade; And then a laughing zany lent him aid. 1848 L. Hunt Jar of Honey vi. 75 Those who had flattered him most when a king, were the loudest in their contempt, now that he was the court-zany. 1883 M. B. Betham-Edwards Disarmed viii, Everybody is good to the Court-fool, the zany!

2. Hence in transf. and allusive uses, with various shades of meaning: a. An attendant, follower, companion, assistant: almost always contemptuous (sometimes, hanger-on, parasite), and with direct reference to sense 1. Now rare or arch.

1601 B. Jonson Ev. Man in Hum. ii. iii. (Qo.), I pray thee be acquainted with my two Zanies [1616 Fol. hang-by's] heere. 1602 Middleton Blurt iii. i. E2, Lady Imperia (the Curtezan's Zani). 1616 R. C. Times' Whistle (1871) 136 Ye Aristippian zanies,‥Leave off at last your poysning honnied speach. 1631 Chapman Cæsar & Pompey iv. i, Protean fortune, and her zany, warre. 1673 Hickeringill Greg. Fr.-Greyb. 50 The Directory, and the geud Covenant, (its zanee). 1746 Francis tr. Hor., Ep. i. xv. 37 A vagrant Zany, of no certain Manger, Who knew not, ere he din'd, or Friend or Stranger. 1746–7 Smollett Advice 181 To shine confess'd her zany and her tool, And fall by what I rose, low ridicule. 1760 H. Walpole Let. to G. Montagu 24 Nov., On the Address, Pitt and his zany Beckford quarrelled. 1817 [see mountebank n. 2]. 1880 Q. Rev. Jan. 14 St. John was not content to be a mere zany, he aspired to rival his master as a wit, and to outstrip him as a libertine. 1911 Athenæum 25 Mar. 343/3 To figure as a zany of a peer.

b. An imitator, mimic; esp. a poor, bad, feeble, or ludicrous imitator. Obs.

1606 Dekker Seven Sins v. 31 An Ape is Zani to a man, doing ouer those trickes‥which hee sees done before him. 1627 Drayton Ep. to H. Reynolds 93 As th' English Apes and very Zanies be Of euery thing that they doe heare and see. 1678 Dryden All for Love Pref. b3b, They are for persecuting Horace and Virgil, in the persons of their Successors.‥ Some of their little Zanies yet go farther; for they are Persecutors even of Horace himself. 1730 Flying Post 22 Dec., Their little Zanies about the Country have learnt their Cant.

c. One who resembles, or acts like, a merry-andrew or buffoon; one who plays the fool for the amusement, or so as to be the laughing-stock, of others. Now rare.

1606 Sir G. Goosecappe iii. i. in Bullen Old Pl. (1884) III. 42 Goe too, you French Zanies you. 1630 Bp. Hall Serm., 2 Tim. iii. 5 Wks. 1808 V. 393 What is a Hypocrite but a Player; the Zany of religion? 1728 Pope Dunc. iii. 206 Oh great Restorer of the good old Stage, Preacher at once, and Zany of thy age! 1790 Wolcot (P. Pindar) Adv. to Fut. Laureat xv, I'll not be Zany to a King, not I. 1846 Eclectic Rev. June 662 Sydney Smith‥was a West-end chapel preacher,‥a lecturer in Albemarle-street, and Zany to Holland-house. 1925 Sunday at Home Nov. 91/2 He went capering about all his tasks with a zany-like glee. 1929 C. Day Lewis Transitional Poem i. 11 A burly wind playing the zany In fields of barleycorn. 1976 G. Langford (title) Death of the early morning hero. Episodes of a zany in love.

d. A fool, simpleton, ‘idiot’. Still dial.

a1784 Johnson in R. Cumberland Mem. (1806) I. 263 The lady asked me for no other purpose than to make a Zany of me. 1847 Tennyson in Ld. Tennyson Mem. (1897) I. 241 The printers are awful zanies, they print erasures and corrections too, and other sins they commit of the utmost inhumanity. 1862 Thackeray Philip iii, Whether Andrew was a genius, or whether he was a zany, was always a moot question. 1897 Watts-Dunton Aylwin iii. i, A heaven for zanies and tom-fools!

B. attrib. passing into adj. a. attrib. That is a zany, or characteristic of a zany; †imitative; clownish; foolish, idiotic. b. adj. Comically idiotic, crazily ridiculous.
The adjectival use of the word (as if an adj. formed with -y1) is now the dominant one. In quot. 1938, ‘simple-minded’ (cf. sense 2d).

1616 Anton Philos. Sat. C2, Like a gorgeous robe, Purl'd ore with natures Ape, and Zany-art. 1618 J. Taylor (Water P.) Pennyles Pilgr. E2, Nor Britaines Odcomb (Zanye braue Vlissis) In all his ambling saw the like as this is. 1869 Blackmore Lorna D. xv, He will make some of your zany squires shake in their shoes. 1890 Hall Caine Bondman ii. ii, Your zany doings have shut every other door against you. 1918 G. B. Shaw in Nation 22 June 308/2 Before Shakespear touched Hamlet there was a zany Hamlet who mopped and mowed. 1938 L. MacNeice I crossed Minch viii. 117 The gardener who was apparently zany, used to level his stick at an obelisk. 1957 Manvell & Huntley Technique Film Music v. 204 Sloppy Jalopy (UPA), a zany cartoon. 1959 House & Garden June 76 Luncheon-mats of the subtlest as well as the zaniest designs. 1978 J. Krantz Scruples xii. 349 Television shows that lean heavily on the brand of humor known as ‘zany’, consisting largely of sight gags and the sight of appealing people making cheerful fools of themselves. 1983 D. Cecil Portrait of Lamb ii. ii. 143 A sympathetic spirit able to appreciate his more characteristic and zany vein of humour.

Hence ˈzanily adv.; ˈzaniness.

1958 S. Ellin Eighth Circle (1959) i. i. 11 In Central Park sea lions barked zanily at the sky. 1960 Sat. Rev. 6 Feb. 13/2 Ginsberg, for all his carefully cultivated (and natural) zaniness, is a writer far above Kerouac. 1966 M. Laurence Jest of God ix. 170 I've‥emptied the crucial and precious capsules out of my window, zanily. 1976 Times Lit. Suppl. 21 May 602/2 His wider appeal owes much more to his modern zaniness and irreverence. 1980 L. Birnbach et al. Official Preppy Handbk. 111/1 Many of these forays into zaniness actually wind up involving mayhem or destruction of property. 1984 Financial Times 25 July 13/6 The play is a staple of the repertory and amateur theatre diet, and while it may not inspire to the zanily surreal heights of See How They Run or Madame Louise, Dighton is certainly a name worth conjuring with. 1985 Listener 21 Mar. 28/3 How could anything with Jerry Lewis not fall into a comic range somewhere between surrealistic zaniness and childish destructiveness?