Green’s Dictionary of Slang

1. to divulge a secret, to confess.

1812		Vaux Vocab. of the Flash Lang. in McLachlan (1964) 233: come it: to divulge a secret; to tell any thing of one party to another.
1824		J. Wight Mornings in Bow St. 97: Hallo! says I, how did you come by it? Mum. Hadn’t a word, you know. Only let him come it now, all about it, and I’m satisfied. Don't like to be done.
1857		J. Archbold Magistrate’s Assistant (3rd edn) 4445: To inform = to come it.
1864, 1867, 1870		Hotten Sl. Dict.
1873		Sl. Dict.
1889		Daily Telegraph n.p.: He heard one of the others say in reply, ‘come it,’ meaning to tell – to be quiet [F&H].
1900		Sporting Times 14 Apr. 2/1: Although I unearth him he will not ‘come it’ about the gate, nor, indeed, upon the much-more-useful subject of the best animile [sic] in his stable.
1930		(con. 1920s) D. Mackenzie Hell’s Kitchen 246: The Other Bird ‘had come it all,’ even to telling who would identify me.
1935		Thieves Slang ms list from District Police Training Centre, Ryton-on-Dunsmore, Warwicks n.p.: Come it: [...] admit.
1936		J. Curtis Gilt Kid 270: He would have to have an understanding with the bogy before he came it. It ought to be pretty easy to come to terms.

2. (Aus./UK Und.) to betray, to inform against.

1839		H. Brandon Dict. of the Flash or Cant Lang. 162/1: To come it – to inform.
1857		‘Ducange Anglicus’ Vulgar Tongue.
1865		Leaves from Diary of Celebrated Burglar 151/1: Yellow Jemmy, when he ‘comed it,’ ‘nosed’ upon me as well as Joe.
1882		Sydney Sl. Dict. (2 edn) 10: Lame Jack [...] blew on Sam who frisked a lobb and the same day came it on Joe for fencing the prad / Lame Jack [...] informed on Sam who robbed a till, and the same day informed on Joe for selling the horse.
1895		Crowe Aus. Sl. Dict. 19: Come It, to inform.
1935		G. Ingram Cockney Cavalcade 108: S’posing that – Yid’s ‘come it’ to the splits – where would we be, eh?
1935		Thieves Slang ms list from District Police Training Centre, Ryton-on-Dunsmore, Warwicks n.p.: Come it: Lay information.