From Latin 'trādere' (to hand over), 'trāns-' + 'dare' (to give) — literally 'give across' to the enemy; kin to 'tradition' and 'treason.'
To be disloyal to; to expose something hidden or reveal a secret unintentionally.
From Middle English 'bitrayen,' from 'be-' (thoroughly) + Old French 'traïr' (to betray), from Latin 'trādere' (to hand over, to deliver, to surrender), from 'trāns-' (across, over) + 'dare' (to give), from PIE *deh₃- (to give). To betray is literally 'to give across' — to hand someone over to the enemy. The same root gives 'tradition' (something handed over through