n. a long, submarine shaped turd that is crapped out before, during, or immediately after sex....beneath the bed sheet covers.
The undercover sub slinks, slithers, and slides across the sheets until it comes into contact with one's sexual partner...causing a multitude of reactions which may include, but are not limited to: excitement, anger, fear, frustration, confusion, bewilderment, angst, mild and/or severe depression.
Last night after I did it with Janie, I swear I heard her shart. No sooner had I smelled the crap, then an undercover sub appeared out of nowhere...spreading across my leg like a wet, stinky rash. It ruined the new bed sheets!