bit on the side

/B IH1 T AA0 N DH AH0 S AY0 D/
noun
  1. 1

    A secondary lover, a mistress.

    โ€œI'm married to Paula, but Jackie, my bit on the side, is far more fun.โ€

Source: https://en.wiktionary.org/wiki/bit%20on%20the%20side