Dictionary / Vocabulary / 陰で悪口を言う
かげでわるくちをいう
to insult someone behind their back
面と向かって誉めるような人は、とかく陰で悪口を言うものだ。
Such men as praise you to your face are apt to speak ill of you behind your back.
人の陰で悪口を言うな。
Don't speak ill of others behind their back.