bad·mouth or bad-mouth 
(bădmouth′, -mou-=…|th=-′)
(bădmouth′, -mou-=…|th=-′)tr.v. bad·mouthed, bad·mouth·ing, bad·mouths or bad-mouthed or bad-mouth·ing or bad-mouths
Informal
Informal
To criticize or disparage, often spitefully or unfairly: "those cross-Atlantic aficionados who persistently idolize the British theater and bad-mouth Broadway" (Benedict Nightingale).