A person who breaks someone's trust or acts disloyally, especially by revealing secrets or switching sides.
From Old French 'betrayer,' composed of 'be-' (intensifier) and 'trair' (to betray), itself from Latin 'tradere' (to hand over or surrender). The concept of one person handing another over to harm became the core meaning.
The word 'betrayer' carries so much weight in literature and history because betrayal requires trust first—you can't betray a stranger, which is why stories from Judas to Shakespeare's Othello are so emotionally powerful.
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