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A person (or persons) who bursts in un-annouced, especially with the intent of catching you while you're masturbating (fapping). sometimes accompanying the surprise entry with a loud shout of "FAP PATROL!!"

Can also refer to a pet who approaches you while masturbating and tries to lick or touch your genitals.

Bonus points if the victim is interupted while ejaculating.
(Joe is on his bed fapping to a porno mag)
(Bob bursts into the room)
Bob: "FAP PATROL!!!"
Joe: "WTF you gaywad!!"
by Fantino Boy April 14, 2010
(n) To burst in to anothers room, unannounced, and yell outloud such lines as "FAP PATROL", or "PUT YO' DICK AWAY". When successful, and awkward situation occurs.
My buddy busted into my room and fap patrol'd me successfully, we did not talk for an hour.
by WARBARLARBL September 26, 2009