betray (v.)

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").

It is attested from 1580s as "unintentionally show a true character;" by 1690s as "indicate what is not obvious;" from 1735 as "reveal or disclose in violation of confidence." In Middle English it sometimes was also bitraish, betrash, from the French present-participle stem. Related: Betrayed; betraying.