Traitorous is the adjective for behaving like a traitor — not loyal to your country, your friends, or what you claimed to stand for. It describes acts, words, plans, and sometimes people: a traitorous letter, a traitorous plot, traitorous officers. It is a formal and old-sounding word, most at home in history books, court language, war stories, and political speeches, and quite rare in ordinary talk. In everyday English people simply say disloyal, or turn the noun into a phrase — "what he did was the act of a traitor". Because it sounds so grand, modern speakers often use it half in fun, about very small betrayals: a traitorous thought when you find yourself enjoying the rival team's win. It is worth putting next to treacherous, which looks like a close cousin and shares the same Latin root but has drifted. Treacherous now most often means dangerous and not to be relied on — treacherous roads, treacherous ice — and only sometimes means disloyal. Traitorous never means dangerous; it stays with betrayal. The stress falls on the first part, "TRAY-tuh-rus".