malicious gossip; backbiting; speaking ill behind someone's back
陰口を叩く人は信用されなくなる。
People who gossip behind others' backs lose trust.
陰口を聞かされて不愉快な気持ちになった。
I felt unpleasant after being told gossip about someone.
本人のいない所で陰口を言うのはやめよう。
Let us stop speaking ill of people behind their backs.