Backstabbing is the act of secretly doing harm to someone, especially by saying bad things about them, in order to gain an advantage. It is the behaviour; the person who does it is a backstabber. Offices, friend groups, and politics are where you hear it most. It is informal and always disapproving. You can also use it like an adjective: backstabbing colleagues, a backstabbing campaign. Gossip can be idle; backstabbing has a purpose — the speaker wants to climb, or to push the other person down. Though actually, people sometimes throw the word at any unkind talk behind someone’s back, even when there is no clear prize, because the feeling of betrayal is the same.