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1.
When a young male masturbates as many times as he can with very little recovery time.
Nerd #1: "Dude, what did you do this weekend?"

Nerd #2: "I had a spunkathon to see how many loads I could pump onto a pair of my sister's panties."

Nerd #1: "DUDE! That's so frickin cool!
by spontaneous spunk December 17, 2011
 
2.
A result of something unexpectedly and exciting happening at an event where a crowd of people have like minded ideals.
Tony the duty manager that day said, "Hey up Steve, watch out for the spunkathon on the end of platform 3 when those nerdy trainspotters see the new class 66 locos emerge from Gaer tunnel".
by swineyvee October 09, 2006