In the act of masturbating to online pornography, when your hand reaches the top of your shaft and slams the desk and creates a thunder-like sound.
person 1: Is it storming outside? I hear thunder but I don't see any lightning.

person 2: Oh that's just John in the next room over thunder fapping. He must be watching some good stuff.
by Spilner April 15, 2011
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When your entire body cramps up from a single masturbation session with the most glorious orgasm you've ever had.
Damn dude, I saw some shit so crazy last night I thunderfapped and spent the morning in the ICU
by Marcus Kraulter November 19, 2011
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