side piece
noun
- part forming the side of something, or attached at the side
Wiktionary
noun
Etymology: Jocular reexpression of the concept of a piece of ass on the side.
- A 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)”
“I was already putting two and two together that I was just a side piece — a booty call.”