Green’s Dictionary of Slang

1. to attack verbally, to slander.

1941		J. Thurber in Sat. Eve. Post 5 Apr. 9/2: He bad-mouthed everybody [OED].
1958		E. Gilbert Vice Trap 110: I’d real bad-mouth him, because I wanted to hurt him.
1969		‘Iceberg Slim’ Mama Black Widow 136: Ah want yu tu stop bad moufin me.
1976		P. Theroux Family Arsenal 165: It’s unlucky to badmouth the dead. Even if they are fuckers.
1982		H. Beaton Outside In I i: Told he the new chick was gettin’ on me nerves. Bad mouthin’ me work.
1983		J. Byrell (con. 1959) Up the Cross 100: It was very much out of character for Wiesbaden Willie to badmouth another German.
1993		I. Welsh Trainspotting 208: It wis oot ay order bad-mouthin Sick Boy like that.
1997		G.V. Higgins Change of Gravity [ebook] And then they’ll go out and start bad-mouthin’ me. Make me look like shit, all over town.
2002		(con. WWII) R. Mooney Father of the Man Prologue: You could be really vile, like he was, with high-society British quiffs, but couldn’t bad-mouth a prostitute.
2002		R.G. Barrett Mystery Bay Blues 156: I will not have you bad mouthing the integrity of Mr Galese.
2005		C. Buzzell My War (2006) 19: Everything he had to say [...] began with a whole lot of bad-mouthing of the [Marine] Corps.
2010		(con. 1973) C. Stella Johnny Porno 216: It had to do with the way she had been badmouthing the kid’s father.
2012		R. Milward Kimberly’s Capital Punishment (2023) 12: I’ve often wondered why people like to [...] badmouth you, or key your car.
2017		G. Gilmore Headland [ebook] ‘Will you not bad-mouth me to know one what knows me’.
2024		J. Meades Empty Wigs (t/s) 394: [Their] divorce had not inhibited their bickering and mutual badmouthing.

2. to beat someone in an argument or verbal contest.

1976

R. Abrahams Talking Black.