When one repeatedly masturbates with both arms until they both give out.
Guy 1: Dude, I can't feel my arms!
Guy 2: Why not?
Guy 3: Marathon Masturbation!
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An exceptionally long episode of masturbation fits. These episodes are usually brought on by sexual deprivation and bordom; however, and I'm not making this up, they have been used a means of fundraising for sexual related causes such as safe sex. A masturbation marathon is a strenuous event that requires disciplined training and preparation. There are several related subcatagories such as time, ejaculation distance, and most orgasms for both male and female competitors (I'm totally serious - google it).
Hillary's return brought an end to Bill's 72 hour masturbation marathon.