Green’s Dictionary of Slang

badmouth adj.

[badmouth n. (3)]

malicious, defamatory.

[US]A. Brooke Last Toke 150: The bad-mouth cop drew his piece, waved it menacingly.
[US]D. Woodrell Muscle for the Wing 65: You really think I’m goin’ to do a bad-mouth testimony on one cop to another?