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1.
Upon seeing a chick so hot, you instantaneously cream your pants. It's like a super-fast wet dream, but awake and in public. Often used in hyperbole since guys don't actually do this, except lame-ass premies.
Guy: Dude, did you see how tight that chick's ass was?

Guy 2: Yeah man. I just instabusted.

************************************

*As you pass a hot girl*

Dude: Instabust!
by SpicyKB October 04, 2007
 
2.
When something so good happens you cant help but creaming your pants instantly.
Man when Sean Dockery hit that 3 as time expired over VT, the whole stadium instabusted!
by The Real Nard Dog March 11, 2011