The involuntary action of manually manipulating one's genitals during REM sleep. This phenomenon is responsible for some unverifiable portion -- scientific estimates point to 5-10% -- of nocturnal emissions. It is rarely discussed in polite conversation, but is assuredly real. When caught in the act by an incidental third party observer, the awoken offender is often characterized by denial and defensiveness. The following sample text is a rare exception.
Brett: <to roommate sleeping in common living area> Mike -- What are you doing?
Mike: <wakes suddenly and looks down at occupied hands> Dude ... I'm trying to sleepsturbate here.
Brett: What? Can you do that in your room?
Mike: I'll spell it out for you later, asshole.