bit on the sideSEX/HAVE SEX WITH British EnglishBrE informal 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 婚外情人，相好〔幽默用法或含贬义〕 She stayed, in the hope that he’d tire of his bit on the side. 她留下来了，希望他会厌倦他的相好。
→ bit on the side at bit1(13)