A Two Time
Having a secret love affair while your significant other is clueless and probably eating a taco at the mall.
He's cheating on me with my best friend. I know it, and I'm going to tell him he's a two-timing fraud!
She had an affair with my uncle and didn't even tell me. That's the worst two-timing I've ever seen!
He had a secret affair with my mom. That's the most two-timing thing I've ever heard of.