A person who betrays or deceives someone who trusted them, especially in a sneaky or underhanded way.
Metaphorical compound from 'backstab' (imagining stabbing someone in the back they can't see), referring to betrayal done without the victim knowing. The term became common in English around the 1800s.
The metaphor of 'backstab' is powerful because it imagines attacking someone in the one direction they can't defend—their back—which is why betrayal by a trusted person feels so much worse than honest opposition!
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