a bit on the side
1. A person who is involved in a sexual relationship with someone who is married. Primarily heard in UK, Australia. I spotted Harry at the diner with a woman much younger than his wife. She must be his bit on the side.
2. Such a relationship. I spotted Harry at the diner with a woman much younger than his wife. I think he's having a bit on the side.
3. Income from an additional job. My wife has started to earn a bit on the side to help us offset all the extra expenses from the renovation.