secret ass 

verb. To have relationships on the side, or to have sexual relations without one's friends having knowlege of the encounters.

noun. The person who acts as the significant other in said events.

Example 1
Friend 1: I haven't seen Pete around on the weekends lately. What the hell is he up to?
Friend 2: I think he's getting some secret ass.

Example 2
It wasn't 'till months later that Pete finally got up the courage to introduce his secret ass to his friends.

secret ass by hardcore_2031 July 15, 2005

Secret Assassin 

Someone, a man specifically, who cannot resist his attraction to sheep.

Beth: Did you hear Mark is a secret assassin?
Kim: NO WAY. I'll have to hide the sheep in my farm!

Secret Assassin by theninjachicki January 23, 2011