British Englishinformalsomeone'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:
She stayed in the hope that he'd tire of his bit on the side.
British Englishinformalsomeone of a lower social class that someone has a sexual relationship with - used humorously
WORD CHOICE: WORD CHOICE: a bit, a bit ofUse a bitbefore an adjective, not before a noun, nor before an adjective+noun• He's a bit shy (NOT a bit shy man). Before a noun or adjective+noun, use a bit of• Let's listen to a bit of music (NOT a bit music). • It was a bit of a strange decision (NOT a bit strange decision).You can also use a bitafter the main verb• I cried a bit (NOT a bit cried).
Definition from the Longman Dictionary of Contemporary English Advanced Learner's Dictionary.