Green’s Dictionary of Slang

a fool, a simpleton; by ext. a cuckold.

1590		Cobbler of Canterbury (1976) 20: A cuckold cried up is a peevish, snappish, quarrelsome ninny-hammer, who so wearies his wife with causeless jealousy, that in the end she gives him cause.
1593		Nashe Four Letters Confuted in Works II (1883–4) 253: Whoreson Ninihammer, that wilt assault a man & haue no stronger weapons.
1599		Nashe To his Readers’ in Lenten Stuffe A4: Thou art a Ninnihammer.
c.1602		Middleton Family of Love V i: I say you are a ninnihammer, and beware the cuckoo.
1611		J. Cook Greenes Tu Quoque Scene ix: scat.: Ninni-hammer.
1618		Owles almanacke 51: [W]omen shall saddle their poore Husbands backes, and make plaine Ninny hammers of Noddies.
c.1629		R. Brome City Wit V i: Why Goodman Fool, you Coxcomb, you Ninnihammer, you Clotpold Countrey Gentleman.
1630		Tinker of Turvey 23: A Cuckold cryed vp, is a peeuish, snappish, quarrelsome Ninny-hammer.
1653		Urquhart (trans.) Gargantua and Pantagruel (1927) I Bk I 103: The bunsellers or cake-makers [...] did injure them most outrageously, calling them [...] jobbernol goosecaps, foolish loggerheads, flutch calf-lollies, grouthead gnat-snappers, lob-dotterels, gaping changelings, codshead loobies, woodcock slangams, ninnie-hammer fly-catchers, noddiepeak simpletons, turdy-gut, shitten shepherds, and other such like defamatory epithets.
1658		J. Cleveland Rustick Rampant 10: No wonder, when Peter the Hermits Goose was believed to be the Holy Ghost, that John amongst as very Ninnyhammers, could strike up for a Prophet.
1663		Head Hic et Ubique IV iv: Opportunities – that any man that had guts in his brains, wu’d have laid hold on. A couple of Ninnyhammers [...] have taken the wrong Sow by the ear.
1680		M. Stevenson Wits Paraphras’d 87: That such nice Dames shou’d for a Ball / Uncase their scutts, and shew you all, / Then send to me to scowr your Rammer? / Don’t think me such an Innyhammer.
1683		Night-Walker of Bloomsbury 2: I’le undertake there’s nere a Booth in Pork-Fair but would have dressed up a Hobgoblin more artificially than such a consultauon [?] of Ninny-Hammers.
1699		N. Ward London Spy VII 155: You Cuckoldly Company Whiffling, Pedling, Lying, Over-reaching Ninny-Hammers, who were forced to desire some Handsome Batchelor to Kiss your Wives.
1712		J. Arbuthnot Hist. of John Bull 30: Have you no more manners than to rail at Hocus, that has saved that clodpated, numskulled, ninny-hammer of yours from ruin.
1725		New Canting Dict. n.p.: Ninny-hammer a silly Senseless Fellow.
1753		Hawkesworth Adventurer n.p.: The words ninnyhammer, noodle, and numscull are frequently bandied to and from betwixt them [R].
1759–67		Sterne Tristram Shandy (1949) 597: And here [...] shall I be called as many blockheads, numsculs, doddypoles, dunderheads, ninny-hammers, goosecaps, jolt-heads, nincompoops, sh--t-a-beds – and other unsavory appellations.
1785, 1788, 1796		Grose Classical Dict. of the Vulgar Tongue.
1800		W.H. Murray Obi; or, Three-Fingered Jack I i: Nonsense! ye black ninny-hammers.
1812		‘Hector Bull-us’ Diverting Hist. of John Bull and Brother Jonathan 65: She called him prating gabbler, liquorish glutton [...] codshead booby, noddipeak simpleton, ninnihammer gnatsnapper, and various other names.
1825		‘All England Are Slanging It’ Universal Songster I 40/1: He must be a ninny hammer / Who cannot hammer flash in him, and patter it without a grammar.
1830		W.H. Williams Wreck I i: Won’t it? but it will, you ninnyhammer.
1834		Ely’s Hawk & Buzzard (NY) 15 Mar. 2/2: You ninnyhammer — you less than a barber.
1840		Boon’s Lick Times (Fayette, MO) 20 June 2/5: The infernal old imbecile! The ninny-hammer!
1849		Morn. Post (London) 17 Feb. 6/5: Dang a’ them Peelites! ninny-hammer Noodles / Led by the noses.
1851		G. Borrow Lavengro III 401: You precious pair of ninnyhammers.
1860		Ashtabula Wkly Teleg. (OH) 18 Feb. 2/5: We have found its editor a ninny-hammer.
1864, 1867, 1870		Hotten Sl. Dict.
1871		Memphis Dly Appeal (TN) 23 mar. 1/1: It was readily granted by the ninnny-hammers who comprise the majority.
1879		C.J. Kickham Knocknagow 12: ‘Blur-an-agers, have sinse, sir – have sinse.’ ‘Have sense yourself – and that’s what you’ll never have, you ninny-hammer,’ retorted the master.
1889		Salt Lake City Herald 31 Oct. n.p.: Did the great Ohio statesman commence his labors by making an ass of himself [in] association with a lot of [...] ninnyhammers?
1896		N. Platte Tribune (NE) 3 July 1/6: He was told by these young ninnyhammers to say [etc.].
1897		‘F. Anstey’ Baboo Jabberjee BA 144: I am no ninnyhammer to consult an exploded astrologer.
1897		‘F. Anstey’ Baboo Jabberjee BA : .
1902		Wash. Times (DC) 5 Oct. 9/1: Lobster - A fink [...] a lunkhead, a ninnyhammer.
c.1914		G. Fitzmaurice Ointment Blue in Plays II (1970) 4: God forgive the laddo! The neighbours laughing at us and we going in a pair ninny-hammers knowing nothing .
1922		(ref. to 1859) Beloit Dly Call (KS) 14 Sept. 8/4: ‘He’s a poor nut [...] In ’59 we ahould have called him a “ninnyhammer”’.
1932		Yorks. Post 18 May 6/2: He may, of course, be a ninnyhammer.