Meaning

verb reveal unintentionally

verb deliver to an enemy by treachery

verb disappoint, prove undependable to; abandon, forsake

originFrom Middle English betrayen, bitrayen (“to commit an act of treason against”), equivalent to be- + tray (“to betray”). Middle English bi- is from Old English be- (“be-”), from Proto-Germanic *bi- (“be-”), from Proto-Germanic *bi (“near, by”), ultimately from Proto-Indo-European *h₁epi (“at, near”). Compare also traitor, treason, tradition. The modern sense “to disclose, discover, reveal…

Etymology from Wiktionary, CC BY-SA 3.0