Moonlighting was the practice of taking a second part-time job (usually at night, hence "moonlight") to supplement one's primary source of income.
In 2364, Madeline, Dixon Hill's secretary, saw that Jean-Luc Picard (as Hill) was still wearing his Starfleet uniform. Unfamiliar with its provenance, she inquired whether he was moonlighting at the Fairmont Hotel as a bellboy. (TNG: "The Big Goodbye")