A backstabber is someone who secretly does you harm, especially by saying bad things about you to other people, so they can get ahead. The picture is old and nasty: stabbing a person in the back, where they cannot see the knife coming. Nobody is talking about a real knife here, though actually the metaphor is so strong that people still feel the shock of it. It is informal, and it is an insult. The person is the backstabber; the secret unkind behaviour is backstabbing. You use it about work, friendship, and politics — anywhere trust was supposed to hold. Saying hard things to someone’s face is not this; the whole point is that it happens behind them.

背後捅刀的人