short for 'Massive Sperm Buildup' - usually occours in young male adults under 25 as a result of no sexual activity for more than a week or 2
Derek had not masturbated for about 3 weeks and was suffering from a severe case of MSB, but after watching 5 minutes of crazy cumshots #4 he had a massive sperm bomb and had to use his blue bandanna to clean up!
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