From Middle Englishbetrayen, betraien, equivalent to be- + tray(“to betray”). English tray(“to betray”) derives from Middle Englishtraien, from Old Frenchtraïr(“to commit treason, betray”), from Latintrādere, present active infinitive of trādō(“deliver, give over”, verb). Compare also traitor, treason, tradition. In Middle English betrayen meant solely "to commit an act of treason against someone; deliver someone treasonably to an enemy; betray one's trust; deceive, mislead". The modern sense "to disclose, discover, reveal unintentionally" is due to influence from or merger with Englishbewray(“to reveal, divulge”), which is similar in sound and meaning. The similarity with Germanbetrügen, Dutchbedriegen, from Proto-Germanic*bidreuganą(“to betray, deceive”), is coincidental.
2012 May 24, Nathan Rabin, “Film: Reviews: Men In Black 3”, in The Onion AV Club[1]:
Jones’ sad eyes betray a pervasive pain his purposefully spare dialogue only hints at, while the perfectly cast Brolin conveys hints of playfulness and warmth while staying true to the craggy stoicism at the character’s core.
1966, Marc Léopold Benjamin Bloch, French rural history:
Again, to take a less extreme example, there is no denying that although the dialects of northern France retained their fundamentally Romance character, they betray many Germanic influences in phonetics and vocabulary, [...]
(transitive) To mislead; to expose to inconvenience not foreseen; to lead into error or sin.
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