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."
by Lifewithaknife October 31, 2011

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Top Definition
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.
by cheesyballs January 26, 2014

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The Urban Dictionary Mug

Lotsa space for your liquids.

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One side has the word, one side has the definition. Microwave and dishwasher safe. Lotsa space for your liquids.

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The Urban Dictionary T-Shirt

Soft and offensive. Just like you.

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The Urban Dictionary T-Shirt

Smooth, soft, slim fit American Apparel shirt. Custom printed. 100% fine jersey cotton, except for heather grey (90% cotton).

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