"hold a second job, especially at night," 1957 (implied in moonlighting), from moonlighter (1954), from the notion of working by the light of the moon. Related: Moonlighting. Earlier the word had been used to mean "commit crimes at night" (1882). The noun meaning "light of the moon" is attested from mid-14c. Moonlit (1830) is first attested in Tennyson.