Side beau
A man a woman dates in secret, in addition to a husband, partner or boyfriend.
Alice: Can you believe that since Jane has been dating John on the side, she has become happier?
Orphelia: Is Jane not married to Charles?
Alice: Yes. John is Jane’s ‘side beau’.
Orphelia: Is Jane not married to Charles?
Alice: Yes. John is Jane’s ‘side beau’.