Breaking someone's trust by revealing their secrets or acting against their interests; revealing something unintentionally.
From Old French 'betray' (be- intensive + tray 'to hand over'), combining Old English 'be-' with a root meaning 'to hand over or deliver.' The modern sense of treachery developed from this.
The phrase 'your face betrays your emotions' shows how the word extended from deliberate betrayal to mean accidentally revealing something—it's like your face 'hands over' your secrets even when you try to hide them!
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