be·tray / biˈtrā/ • v. [tr.] be disloyal to: his friends were shocked when he betrayed them. ∎  be disloyal to (one's country, organization, or ideology) by acting in the interests of an enemy: he could betray his country for the sake of communism. ∎  treacherously inform an enemy of the existence or location of (a person or organization): this group was betrayed by an informer. ∎  treacherously reveal (secrets or information): many of those employed by diplomats betrayed secrets and sold classified documents. ∎ fig. reveal the presence of; be evidence of: she drew a deep breath that betrayed her indignation.DERIVATIVES: be·tray·al / -əl/ n.be·tray·er n.

betray give up treacherously XIII; reveal involuntarily XVI. f. BE- +†tray (XIII) — OF. traïr (mod. trahir) — L. trādere deliver up (see TRADITION).
Hence betrayal XIX; see -AL2.