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").

The earliest known use of the verb betray is in the Middle English period (1150—1500). OED's earliest evidence for betray is from around 1250, in the writing of Layamon, poet. betray is formed within English, by derivation.