early 13c., bitraien, "prove false, violate by unfaithfulness;" c. 1300, "deliver or expose to the power of an enemy by treachery," also "mislead, deceive, delude," from be- + obsolete Middle English tray, from Old French traine "betrayal, deception, deceit," from trair (Modern French trahir) "betray, deceive," from Latin tradere "hand over," from trans "across" (see trans-) + dare "to give" (from PIE root *do- "to give").

Word History Etymology Middle English, from be- + trayen to betray, from Anglo-French trahir, from Latin tradere — more at traitor