- He makes a little money on the side by cleaning windows in his spare time.
- I think he has another woman on the side (=a relationship with a woman who is not his wife).
- I'd like a salad with the dressing on the side (= with the dressing served separately from the salad), please.
- I'll have a omelette with fries on the side, please.
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△ in addition to your main job.
△ [mainly US] (of food in a restaurant) served on another plate, or not on part of the meal.