Disloyal

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late 15th century: from Old French desloial, from des- (expressing negation) + loial ‘loyal’.

wiktionary

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Middle English, from Anglo-Norman desleal, desloial

etymonline

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disloyal (adj.)

early 15c. (implied in disloyally), "not true to one's allegiance" (to a sovereign, state, or government), from Old French desloial, desleal "treacherous, false, deceitful" (Modern French déloyal), from des- "not, opposite of" (see dis-) + loial "of good quality; faithful; honorable; law-abiding; legitimate, born in wedlock," from Latin legalem, from lex "law" (see legal). Sometimes also "not true to one's obligations or engagements," especially to a lover, spouse, or friend, (late 15c.), but this sense is rare.