loyal (adj.) Look up loyal at Dictionary.com
"true or faithful in allegiance," 1530s, in reference to subjects of sovereigns or governments, from Middle French loyal, from Old French loial, leal "of good quality; faithful; honorable; law-abiding; legitimate, born in wedlock," from Latin legalem, from lex "law" (see legal).

Identical with legal, which maintains the Latin form; in most uses it has displaced Middle English leal, which is an older borrowing of the French word. For the twinning, compare royal/regal. Sense development in English is feudal, via notion of "faithful in carrying out legal obligations; conformable to the laws of honor." In a general sense (of dogs, lovers, etc.), from c. 1600. As a noun meaning "those who are loyal" from 1530s (originally often in plural).