How to say "bad-mouth" in Japanese

• 悪く言うわるくいう
  expression, godan verbCovers the general sense of bad-mouth: to speak badly or ill of someone. A less common rendering; typical for unfavorable or critical talk about a person that need not amount to outright insults.
• 悪口を言うわるぐちをいう
  expression, godan verbCovers bad-mouth in the sense of insulting someone: to say something insulting about a person. A less common rendering; typical for talk whose content is openly insulting rather than merely unfavorable.

Both are less common ways to render bad-mouth: choose 「悪く言う」 for speaking ill of someone in general, and 「悪口を言う」 when what is said amounts to insulting the person.

Usage comparisons are AI-assisted and cross-checked by multiple models; for study reference only

• 悪く言うわるくいうto speak badly (of)
• 悪口を言うわるぐちをいうto insult