Green’s Dictionary of Slang

1. (also gills) a gullible person.

1592		W. Warner Albion’s England Bk 7 xxxvii n.p.: The simplest Gill or Knave [F&H].
1600		J. Day Blind Beggar of Bednall-Green Act I: Ha, ha, ha, gill, gill, gill, I have been ready to burst. Son pray thee tell me how thou laid’st this plot?
a.1790, 1809		implied in rum gill under rum adj.
1812		Vaux Vocab. of the Flash Lang. in McLachlan (1964) 243: gill a word used by way of variation, similar to cove, gloak or gory; but generally coupled to some other descriptive term as a flash-gill, a toby-gill.
1879		Cincinnati Enquirer 7 Sept. 10/7: Gill – The man who backs the ‘snaps’ – a party with a few dollars, no brains and less experience in the show business. He always returns a sadder, wiser, broke-up man, his shekels gone, and the only satisfaction left of being added as one to the number of gills.
1887		Daily Trib. (Bismarck, ND) 23 Oct. 4/1: Those people who [...] haunt stage doors [...] are ‘gillies,’ ‘gills,’ ‘guys,’ ‘chappies,’ ‘mashers,’ ‘chumps,’ ‘lunkheads’ and ‘lugs.’.
1927		‘Jargon of the Und.’ in DN V 447: Gill, A credulous person.
1929		H.C. Witwer Yes Man’s Land 307: Jack and Maxim had put the fear of the Lord into this double-crossing gil and he was now house broke.
1934		G. & S. Lorimer Stag Line 161: I hadda loan Mosey my roll to flash on the gills at the Carny.
1953		B. Behan Scarperer (1966) 106: Your gills here, he says he doesn’t understand English.

2. (also gillie) a general term for a man.

c.1811		‘A Leary Mot’ in Farmer Musa Pedestris (1896) 77: Now Moll’s flash com-pan-ion was a Chick-lane gill, and he garter’d below his knee.
1821		‘An Amateur’ Real Life in London I 596: You will here have to mingle with some of the queer Gills and rum Covies of all ranks.
1830		W.T. Moncrieff Heart of London II i: I haven’t a downier gill in the whole college than you are.
1848		Flash Dict. in Sinks of London Laid Open.
1874		Cairo Bull. (Cairo, IL) 5 Nov. 2/3: [from The Graphic, London] Farewell, gonnoffs and gnostics all, / And gillies sweet and free.
1923		H.C. Witwer Fighting Blood 141: I’ve tried everything I know to get this gil to fight us and no can do!
1929		T. Thursday ‘And Howe’ in Everybody’s Feb. 🌐 That gil you wished on me would of been okay lecturing at Yale and Harvard.
1930		Roosevelt Sykes ‘Papa Sweetback Blues’ 🎵 He don’t work, he call himself a smart gill, / He don’t care about nobody.

3. an interfering person.

1823

‘Jon Bee’ Dict. of the Turf, the Ring, the Chase, etc. 87: A Gil is he who pokes in his nose unasked, as if authoritatively. Gil Gill, was a beadle of Farringdon-within.

4. (UK Und.) oneself.

1893

P.H. Emerson Signor Lippo 47: We could have the small room with two letties to our own gills.