on the side

Từ điển Oxford Advanced Learner 8th

on the ˈside idiom
(informal)
1. in addition to your main job
•a mechanic who buys and sells cars on the side
2. secretly or illegally
•He's married but he has a girlfriend on the side.
3. (especially NAmE) (of food in a restaurant)served at the same time as the main part of the meal, but on a separate plate
Main entry: ↑side_idiom