Talking maliciously about someone behind their back; backbiting or gossip meant to damage reputation.
職場で陰口を叩かれていると知って、彼は深く傷ついた。
When he learned people were talking behind his back at work, he was deeply hurt.
陰口を言うことで一時的に気が晴れても、人間関係は悪化することが多い。
Even if gossiping makes you feel better for a while, it often worsens relationships.
陰口は聞き流すのが一番だと言われた。
I was told it's best to ignore gossip.