Entry 1 sense Webster, 1913 Treachery /trĕch'-ẽr-ē/ · Treach·er·y · IPA /ˈtɹɛt͡ʃəɹi/ 01 n. Violation of allegiance or of faith and confidence; treasonable or perfidious conduct; perfidy; treason. 1. Violation of allegiance or of faith and confidence; treasonable or perfidious conduct; perfidy; treason. “Be ware, ye lords, of their treachery.” — Chaucer. “In the council chamber at Edinburgh, he had contracted a deep taint of treachery and corruption.” — Macaulay.