Same word as badmouth:

backbite, bad-mouth, bash, belittle, berate, blow off, calumniate, cap, castigate, cuss out, cut down, cut to the quick, decry, defame, derogate, discount, do a number on, dump on, give a black eye, hurl brickbat, insult, knock, minimize, nag, offend, oppress, persecute, pick on, put down, rag on, reproach, revile, ride, rip up, run down, scold, signify, slam, slap, sling mud, smear, sound, swear at, tear apart, trash, upbraid, vilify, vituperate, zing