a bit on the side
if someone has a bit on the side, they are involved in a sexual relationship with someone who is not their usual partner
bit on the side
someone's bit on the side is a person they are having a sexual relationship with, even though they already have a wife, husband, or partner – used humorously or to show disapproval

[TahlilGaran] Dictionary of Contemporary English ▲

bit on the side
[British & Australian]
if someone has a bit on the side, they are involved in a sexual relationship with someone who is not their usual partner.
He had a bit on the side for years until his wife found out.

I knew she'd never leave her husband for me. I was just her bit on the side.

[TahlilGaran] English Idioms Dictionary ▲