Green’s Dictionary of Slang

1. a eunuch; an impotent man.

1592		Greene Defence of Conny-Catching 55: Marian with a sharpe rasor cut off his stones. [...] Thus was this lustie cocke of the game made a capon.
1594		R. Carew (trans.) Huarte’s Examination of Men’s Wits 279: Of a 1000 such capons who addict themselues to their booke, none attaineth to any perfection, and euen in musicke (which is their ordinarie profession).
1613		G. Stephens Cinthias Revenge IV vi: I can stab you (capon) to the quicke, Cut off your Eunuch-nose.
1622		Massinger Virgin-Martyr II i: Would he that first tempted me to have my shoes walk upon Christian soles, had turn’d me into a capon, for I am sure now the stones of all my pleasure, in this fleshly life, are cut off.
1633		Massinger Guardian IV i: If he prove not A Cock of the Game, cuckold him first, and after Make a Capon of him.
1674		Fryer New Account of East India II 88: The Moors are by Nature plagued with Jealousy, cloistring their Wives up, and sequestring them the sight of any besides the Capon that watches them.
1689		R. L’Estrange Erasmus Colloquies 116: They are no Capons, I’ll assure you [...] but may very probably be called Fathers.
1728		Trip through London 50: [An] old Woman [...] was falling into a Discourse about Husbands, Capons, and Marrow-bones.
1746		W. Dunkin Parson’s Revels (2010) 62: For Spurs [...] / Were made for Birds of mettle Keen, / And not for Dunghill-Cocks, I ween, / Or Capons.
1785, 1788, 1796		Grose Classical Dict. of the Vulgar Tongue.
1811		Lex. Balatronicum.
1823		[C.M. Westmacott] Mammon in London 1 331: You’d think [...] that the summum bonum of existence was [...] a eunuch’s voice [...] what turgid bulletins if the capon happens to be hoarse.
1833		‘Lucy and Her Music Master’ in Convivialist in Spedding & Watt (eds) Bawdy Songbooks (2011) IV 16: No Capon, I assure you, sirs, no Signor velluti, / For none but of true English breed would ever do for Lucy.

2. used for a variety of fish, e.g. a red herring, a sole, a dried haddock [the cheap herring substituted for the more expensive bird].

c.1640		J. Smyth Hundred of Berkeley (1885) 319: The sole wee call our Seuverne Capon [F&H].
1719		A. Ramsay Hamilton II iii: A Glasgow capon and a fadge ye thought a feast.
1770		D. Gunston (ed.) Jemmy Twitcher’s Jests 80: They give new names to ev’ry dish / [...] / For haddocks are call’d capons there [i.e. Buckhaven, Scotland].
1812		W. Tennant Anster Fair (1815) II xxi 45: Each to his jaws / A good Crail capon* holds. [...] (*A Crail capon is a dried haddock).
1867		W.H. Smyth Sailor’s Word-Bk (1991) 159: Capon. A jeering name for the red-herring.

3. a young homosexual man, sometimes but not invariably a prostitute [the ref. is to chicken n. (4d) rather than any implied sexual malfunction].

1936		D. Burley in Chicago Defender 29 Feb. 11: A certain capon ran off with a society girl’s P.O. husband and won’t let him come back!
1944		‘Jiver’s Bible’ in D. Burley Orig. Hbk of Harlem Jive.
1966		Trimble 5000 Adult Sex Words and Phrases.
1970		San Diego Sailor 74: He sees me out with a young and beautiful sailor and he knows I’m no capon.