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1.
To unload large amounts of sperm into your hand and slap someone with it.
Yo man, I just SpiderDunked Zach today! He had cum hangin' off his face!
by Zach and Charlie December 12, 2005
 
2.
To aggressively dunk cookies in a cup of milk.
"Bitch please! I can spider-dunk my oreos faster than your pussy ass!"
by italianoman March 16, 2009