Slang. to speak critically and often disloyally of; disparage: Why do you bad-mouth your family so much?
Also, badmouth.
Origin: 1935–40; orig. a curse, spell (the sense recorded in Gullah); cf. Vai (Mande language of Liberia and Sierra Leone) dà nyà mà curse, lit., bad mouth
bad·mouth or bad-mouth (bād'mouth', -mouth') tr.v.
bad·mouthed or bad-mouthed, bad·mouth·ing or bad-mouth·ing, bad·mouths or bad-mouthsInformal To criticize or disparage, often spitefully or unfairly: "those cross-Atlantic aficionados who persistently idolize the British theater and bad-mouth Broadway"(Benedict Nightingale).