When you think your buddys gone so you try to get a quick beat off, but they suddenly bust in on you while your tryin to whack it and you try to act like your sleepin, like nothin was goin on.
I left the house only to return to catch bakers ass tryin to sneak-a-beat to some soft core porn on showtime.
The act of secretly and quickly masturbating whilst your parents are home and/or at the grocery store.
"Bye mom, take your time at the grocery store! I'll be fine."
"Heheheh, time to sneak-a-beat!"