Green’s Dictionary of Slang

1. (US campus) to bluff a recitation.

1835		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].
1856		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.

1827		Egan Anecdotes of the Turf, the Chase etc. 71: But the eloquence of Mr Philipps [...] betrayed too much ‘mouthing’.
1849		Bell’s Life in Sydney 14 July 1/3: Hurrah! for those spouters who mouth it a bit.
1853		W.G. Simms Sword and the Distaff 303: Pshaw! what are you both mouthing about? You’re both of you drunk.
1929		E. Dahlberg Bottom Dogs 233: Morris pitched rite in after the kid who was mouthin’ that dirt.
1938		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.
1965		H. Rhodes Chosen Few (1966) 81: She won’t be able to take her mammy mouthin’ at her for long.
1981		W.D. Myers Hoops 95: I [...] listened to her mouth about why I hadn’t met her before when I said I would.
1997		N. Barlay Curvy Lovebox 30: Nood wouldn’t be mouthin’ like that in Kingsley’s face.
2006		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.

1847

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

4. (Aus.) to quieten [? misinterpretation of previous sense].

1895

C. Crowe Aus. Sl. Dict. : Mouthing,‘he wants mouthing,’i.e. he speaks too plainly, and needs curbing in his tongue.

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

1995

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