To harm someone by attacking them from behind, either literally with a weapon or figuratively by betraying their trust when they're vulnerable.
Back + stab; 'stab' from Middle Dutch 'stob' or German 'staupe.' The figurative meaning (betrayal) became dominant in English by the 16th century.
The word is actually older than its figurative meaning—when betrayal became metaphorically called 'backstabbing,' it shows how a physical act became the ultimate symbol of treachery, probably because it's the most cowardly form of attack!
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