Masturbation muscles 

A person who attains muscular definition or gain in the arms primarily through masturbation. One must not uniformly masturbate or masturbate solely with one arm if he is to achieve a significant or aesthetically pleasing result. Ways to fully utilise this workout include but are not limited to:
Set arm days
Switching arms halfway through masturbation
Freestyle (recommended for experienced users only)

''Wow your arms are so big you must workout loads!'' girl says flatteringly while feeling on arm.

''Not that much to be honest with you I'm just naturally endowed,'' typical lie said by happy user of MM method.

''Those are definitely masturbation muscles,'' murmurs a well-educated onlooker to his friend enviously.

Masturbation muscles by cheesyballs January 26, 2014

Masturbation Muscle 

When a male has fapped so much, that one arm is bigger than the other one.

Guy 1 "Dude how did Daniel get so buff?"

Guy 2 "Look at his arm."

Guy 2 "It's because he tones his Masturbation Muscle."

Daniel "No! I just workout."

Masturbation Muscle by Lifewithaknife November 1, 2011

Masturbating Muscle 

The muscle you develop between your forearm and bicep on the arm of the hand you masturbate with.

John: Hey broski, wanna hang tonight?

Joe: Ehh... I dunno, I was really gonna work on toning my masturbating muscle tonight.

John: Oh.. I getcha, I worked on it last night.

Masturbating Muscle by aaronthejoo May 11, 2011