'badmouth' originates from English, specifically the words 'bad' and 'mouth', where 'bad' meant 'not good/negative' and 'mouth' (from Old English 'mūþ') meant 'mouth' or figuratively 'to speak'.

'mouth' developed from Old English 'mūþ' (meaning 'mouth' as noun) and later was used as a verb meaning 'to form words with the mouth' or 'to speak'; the compound 'bad' + 'mouth' (to speak badly of) arose in modern English as a colloquial compound and was recorded in the 19th–20th century English usage.

Initially the components referred literally to 'bad' + 'mouth' (mouth as organ or act of speaking), but over time the compound came to mean specifically 'to speak ill of someone' or 'to disparage', the specialized sense used today.