Green’s Dictionary of Slang

1. a braggart, a boaster, also attrib.

1889–90		Barrère & Leland Dict. of Sl., Jargon and Cant.
1913		Sport (Adelaide) 26 Dec. 8/1: Milda S. and big mouth Trixie are going the pace .
1926		Wood & Goddard Dict. Amer. Sl.
1936		(con. 1920s) Dos Passos Big Money in USA (1966) 751: Pokerface stories told sideways out of the big mouth (from Missouri who took no rubber nickels).
1938		J. Curtis They Drive by Night 229: The old-timer drove all right too, even if he was a bit of a big-mouth.
1943		R. Llewellyn None But the Lonely Heart 295: Big mouth. That’s all He is.
1949		I. Shulman Cry Tough! 56: Shut up! [...] Quiet, big mouth.
1951		Cusack & James Come in Spinner (1960) 266: Cripes, what a mug he’d been, just another one of those big mouths he secretly despised.
1959		F. Norman Fings I i: Tosher comes in. He is the ponce, wide boy, big mouth, coward, humorist, flash dresser, all in one.
1965		H. Rhodes Chosen Few (1966) 94: Oh, I should’a know’d it was you, big mouth.
1985		N. Pileggi Wiseguy (2001) 188: See, big mouth!
1999		Guardian G2 28 June 4: He’s known as a laddish big mouth.
2015		Guardian G2 3 July 6/1: I swear that bitch [...] got a big mouth.
2018		T. Pluck ‘Hot Rod Heart’ in Life During Wartime 100: ‘My Stude’s twice the car your bigmouth son’s is’.

2. an informer, a tell-tale.

1934		Z.N. Hurston Jonah’s Gourd Vine (1995) 47: Hope ole big-mouf M’haley don’t come pukin’ her guts ’round heah.
1948		P. Kavanagh Tarry Flynn (1965) 59: I wouldn’t like to think of you knocking around Dillon’s house, not that I’d ever believe you’d do anything, but you know the big-mouths that’s about this place.
1965		R.L. Pike Mute Witness (1997) 69: I don’t suppose you know that Mr Big-Mouth Chalmers has been screaming to the Mayor, the Commissioner, and the Chief, do you?
1978		R.D. Pharr Giveadamn Brown (1997) 143: ‘Spread the word. Put it in the eer of some bigmouth’.
1989		Beano Comic Library No. 176 52: A grass, eh? A tittle-tattle, eh? A big-mouth, eh? We don’t like tell-tales!
1996		J. Byrell Lairs, Urgers & Coat-Tuggers 91: ‘An aleck - or smart aleck - can be downright dangerous. One - they’re bigmouths. Two - they will shelf you or dob you in at the drop of a hat’.
1997–2000		Da Bomb 🌐 3: Big mouth: A person who talks a lot, especially when discretion is preferred.

3. empty boasting, showing off.

1940		R. Chandler Farewell, My Lovely (1949) 199: I didn’t have any idea of getting tough in the first place except just the routine big mouth.
1953		Kramer & Karr Teen-Age Gangs 24: We try to make plans and all we get from you is a lot of big mouth.
1963		A. Baron Lowlife (2001) 37: So I was stuck with it. My big mouth.
1977		W.D. Myers Mojo and the Russians 76: Kwami and his big mouth.