Green’s Dictionary of Slang

mouth v.

1. (US campus) to bluff a recitation.

[US]J. todd Student’s Manual 115: Should you allow yourself to think of going into the recitation-room, and there trust to ‘skinning,’ as it is called in some colleges, [...] or ‘mouthing it,’ as in others [DA].
[US]B.H. Hall College Words (rev. edn) 328: mouth. To recite in an affected manner, as if one knew the lesson, when in reality he does not.

2. to insult, to criticize, to speak insolently.

[UK]Egan Anecdotes of the Turf, the Chase etc. 71: But the eloquence of Mr Philipps [...] betrayed too much ‘mouthing’.
[Aus]Bell’s Life in Sydney 14 July 1/3: Hurrah! for those spouters who mouth it a bit.
[US]W.G. Simms Sword and the Distaff 303: Pshaw! what are you both mouthing about? You’re both of you drunk.
[US]E. Dahlberg Bottom Dogs 233: Morris pitched rite in after the kid who was mouthin’ that dirt.
[Aus]X. Herbert Capricornia (1939) 67: He mouthed about Elbert, then remembered Oscar and began to mouth about him, explaining why he had called him Uncle Oscar.
[US]H. Rhodes Chosen Few (1966) 81: She won’t be able to take her mammy mouthin’ at her for long.
[US]Eble Campus Sl. Spring 6: quit mouthing – be quiet.
[UK]N. Barlay Curvy Lovebox 30: Nood wouldn’t be mouthin’ like that in Kingsley’s face.
[NZ]P. Shannon Davey Darling 63: She was beside herself, mouthing how she didn’t know how she could’ve ended up with such a useless, angry bastard of a husband.

3. to hit in the mouth.

[Aus]Bell’s Life in Sydney 13 Feb. 2/2: He [...] momentarily foiled the perspicuity of Ike, by mouthing him.

4. (W.I.) to make fun of.

[WI]Francis-Jackson Official Dancehall Dict. 34: Mout to make fun of.

In phrases

mouth off (v.) (orig. US)

1. to speak impudently.

[US]H. Ellison ‘We Take Care of Our Dead’ in Deadly Streets (1983) 61: You’re in bad enough without mouthin’ off to me.
[US]Male mag. in H.S. Thompson Hell’s Angels (1967) 72: We’d go into a bar and someone’d mouth off or try to move in on our chicks and then we’d fight.
[US]L. Heinemann Close Quarters (1987) 240: He [...] mouthed off to some chicken colonel, he said, and was on the next plane.
[Aus]R.G. Barrett You Wouldn’t Be Dead for Quids (1989) 173: Nothing would have happened if you hadn’t of started mouthing off.
[US]J. Wambaugh Secrets of Harry Bright (1986) 58: The hard hats started mouthing off and one thing led to another.
[US]G. Sikes 8 Ball Chicks (1998) 108: She could be beaten by her boyfriend for any number of infractions: not showing up when she was supposed to, mouthing off, refusing to give in to sex.
[UK]G. Malkani Londonstani (2007) 55: I jus mouthin off cos I got me a high sex drive.
[US]A. Steinberg Running the Books 138: I was a kid who hung out with the wrong people, mouthed off.

2. (also mouth on) to boast, to brag.

[US]Milner & Milner Black Players 228: A simple pimp [...] who was ‘mouthing off’ his physical dominance over his ho.
[NZ]H. Beaton Outside In I ii: You forgot yourself! But then it ain’t nothin’ to be mouthin’ off about, is it?
[UK]N. Cohn Yes We have No 209: He started mouthing off.
[UK]D. Mitchell Black Swan Green 313: Anglesey’d been mouthin’ on how he’d hunt down Red Rex.