early 13c., "prove false, violate by unfaithfulness;" c. 1300, bitrayen, "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").
From 1580s as "unintentionally show a true character;" 1690s as "indicate what is not obvious." From 1735 as "reveal or disclose in violation of confidence." Sometimes in Middle English also bitraish, betrash, from the French present participle stem. Related: Betrayed; betraying.