Disloyal means failing to stand by those you owe faith to. A disloyal friend, disloyal to the party, accused of being disloyal.
It is a heavy charge because loyalty is owed rather than earned — you can only be disloyal to something you were bound to. It takes to. It is used constantly in politics, where calling a colleague disloyal is a way of ending an argument about whether they were right. Note disloyalty; and that loyal comes from the Latin for lawful, which is where the sense of obligation comes from.
1adjectivedisloyalFailing to stand by those you owe faith to.
“He felt disloyal for looking for a new job behind his boss's back.”
synonyms: treacherous, faithless
disloyal is a quality of faithlessness.
Anglo-Norman desleal is des- plus leal, from Latin legalis, lawful. Loyal, legal and leal are the same word, so disloyalty is a kind of lawlessness.
Middle French
loyal
English
loyal
English, today
disloyal
From Middle English -, from Anglo-Norman - desleal, desloial, equivalent to dis- + loyal. loyal Borrowed from Middle French loyal. Middle French loyal, from Old French loial, leial, leal, from Latin lēgālis. a doublet of legal and leal.
Etymology from Wiktionary, CC BY-SA