Vocabulary
陰で悪口を言う
かげでわるくちをいう
to insult someone behind their back
expression
verb
Example sentences
面と向（めんとむ）かって誉（ほ）めるような人（ひと）は、とかく陰（かげ）で悪口（わるぐち）を言（い）うものだ。
Such men as praise you to your face are apt to speak ill of you behind your back.