alligator n.

"an animal resembling a serpent, with legs added to it" [Johnson], late 14c., lusarde, from Anglo-French lusard, Old French laisarde "lizard" (Modern French lézard), from Latin lacertus (fem. lacerta) "lizard," a word of unknown origin. The ending in French and English is probably influenced by words in -ard.

It is identical to Latin lacertum "upper arm, muscular part of the arm, from the shoulder to the elbow" (opposed to bracchium), which suggests a pattern similar to that of Latin musculus "a muscle," literally "little mouse" (diminutive of mus "mouse"), so called because the shape and movement of the biceps were thought to resemble mice. It is unclear which Latin sense, the arm-muscle or the lizard, is original. De Vaan finds the words perhaps connected to Greek likertizein "to jump, dance," which Beekes finds likely from Pre-Greek.

Run fast, stand still. This, the lesson from lizards. [Ray Bradbury]

French masc. definite article (including the old neuter), fem. la, from Latin ille "he, that," used in Late Latin and Medieval Latin as the definite article. Cognate with Spanish el. Latin ille "that," illa "by that way, there," replaced Old Latin olle/ollus, perhaps by analogy with iste [de Vaan]; from PIE *hol-no- "that, yonder."

1844, colloquial shortening of alligator.