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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. Symptoms: 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.
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