badmouth

/ˈbædˌmaʊθ/
verb
  1. To say unpleasant or critical things about someone or something.
    • It's not nice to badmouth your coworkers behind their backs.
    • He was fired for badmouthing the company on social media.
    • She badmouthed the restaurant before even trying the food.
Antonyms
What does "badmouth" mean? | whatsthatwordmean | whatsthatwordmean