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When ones main hand becomes stiff and starts to twitch out of control. Chronic masturbation is the leading cause of this common teenage male disease.

Sudden urge to relieve one self
Bulging fore-arm muscle, with inverse proportion to the opposite fore-arm.

If one finds another person/persons with "Mastylock", it is highly recommended to stand back and then state clearly who currently has it and what it is.

Warning: Staring at someones "Mastylock" can sometimes cause the looker to catch "Mastylock", depending on ones sexuality.
"Oh my god Kieran has Mastylock! Its when someone masturbates so much thier hand locks up!"

Jac is a faggot.
by bob schumacer March 17, 2011
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