(v) Informal To criticize or disparage, frequently spitefully or unfairly: "those cross-Atlantic aficionados which persistently idolize the Brit movie theater and bad-mouth Broadway” ( Benedict Nightingale).
(v) To criticize or malign, specially unfairly or spitefully.
(v) speak unfavorably about
badmouth in Community Dictionary
To criticize or disparage, usually spitefully or unfairly. by Yvonne MerrowReport definition