UK:*UK and possibly other pronunciationsUK and possibly other pronunciations/ˈbædmaʊθ/US:USA pronunciation: respellingUSA pronunciation: respelling(badmouth′ or, sometimes, -mouᵺ′)

WordReference Random House Learner's Dictionary of American English © 2020
ˈbad-ˌmouth or ˈbadˌmouth, v. [ + obj]
  1. to criticize, often disloyally:disliked her boss, but never bad-mouthed him.

WordReference Random House Unabridged Dictionary of American English © 2020
bad-mouth  (badmouth′ or, sometimes, -mouᵺ′),USA pronunciation v.t.,  -mouthed, -mouth•ing. 
  1. [Slang.]to speak critically and often disloyally of;
    disparage:Why do you bad-mouth your family so much?Also,  badmouth′. 
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′er, n. 

Collins Concise English Dictionary © HarperCollins Publishers::
bad-mouth vb
  1. (transitive) slang to speak unfavourably about
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