side piece

plural side pieces

1. slangA person with whom someone already in a relationship carries on a physical affair (not emotional affair) without having the intention of forming a serious relationship. Not gender specific.

   Near-synonyms: (feminine) mistress, side chick, side bitch (vulgar)

   3 more examples

   I was already putting two and two together that I was just a side piece — a booty call.

   2004, Wendy Williams, Karen Hunter, Wendy's Got the Heat, page 104

   You basically told him you're cool with being his side piece as long you're number one.

   2008, Lenora Adams, Baby Girl, page 105

   I'm just tired of being treated like a side piece and not a wife.

   2014, Shelly Ellis, The Best She Ever Had, page 246

   Synonyms: bit on the side, mistress, side chick, side bitch