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2.
1. The act of masturbating under another's bed/ in their closet then scaring them.

2. The act of masturbating to a monster character(s).

3. To dress up like a monster and then proceed to masturbate.
1. Charlie pranked his roommate Doug by way of monsterbation but felt embarrassed when he realized that he had scared Doug's girlfriend.

2. Charlie has a monster fetish and has been caught in the act of monsterbation.

3. After a night of failed attempts with women on halloween, Charlie decided monsterbation in his zombie costume was the only way he'd get some.
by m.fucci November 04, 2009
 
1.
The act of masturbating while going "RAAAAAAWWWRRR!" Like a monster/dinosaur
Max decided to experiment with monsterbation, but the noise made his mom come to his room, and discover what he was doing.
by Maudlin La An March 31, 2010
 
3.
when one engages in the touching of oneself at a time when one's significant other is right beside them.

the monster replaces master because one who subjects themselves to such acts are obviously dirty pigs.
olive engages in monsterbation 24/7, even when he has just 69ed with the finest bitch, hence he is the ultimate perve who no one likes.
by Arntnok February 02, 2008
 
4.
Where you jerk off under someone's bed to scare them. Upon finishing, you shout "HAPPY HALLOWEEN" in your scariest voice.
"Hey, Jimmy, what did you get up to for Halloween. Costume party?"
"No, I just went on a monsterbation spree. Scared the little bastards senseless. It was both a trick and a treat."
"This friendship is over."
by Bruno Strongarm March 17, 2014