What it means
To moonlight is to work a second job on the side, usually outside your normal hours and often a bit low-key. It doesn't automatically mean anything dodgy. Most of the time it just means someone's got their main job by day and another little money mission after that.
Usage examples
"I thought Priya was skint, but she’s moonlighting as a nail tech after hours, then legging it home before anyone from work clocks her."
"Mark has been moonlighting as a wedding DJ since November, makes more in one Saturday night than in a whole week at the office, and nobody upstairs has a clue."
"Our neighbour moonlights as a dog walker on Sunday afternoons, takes six dogs round the local park and comes back smelling of fox and contented exhaustion."
"Jen's been moonlighting as a bartender on Fridays, so if she looks half-dead in the Monday meeting, that's the plot twist."
"He was moonlighting as a locksmith for months and only got rumbled when his manager spotted him doing emergency callouts in the next town over."
Where it comes from
It comes from the image of doing extra work by moonlight after the regular working day was done. In the late 1800s it could also hint at secret nighttime activity, but over time it settled into the everyday sense of having a second job on the side.
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