When you jack off for a really long time, and some of the cum that's will eventually burst out just sorta sits on the tip of your dick. Prebust is normally clear, not white.
Jack: Dude, I have sooooooo much prebust on my tip!

John: Well you wouldn't if you weren't jacking off for so long!

Jack: But it's the Animal Channel. It's the only porn I'm allowed to watch!
by Rami Laham February 26, 2008
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