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1.
coming down from a "high" on drugs. usually where the really good feelings start fading away, and you get tired.
I was thizzin allll night, but after a while i started crashing, and went home.
by Heidi Terra January 08, 2007
 
2.
The effect you get when a stimulative substance (Drugs, alcohol, caffeine) wears off. When crashing, you'll suddenly feel very tired and/or sad.
I started crashing hard at last night's party, so I went home.

I really crashed, but I was all out.
by teraku January 18, 2013
 
3.
attending an event or outing which has not been publicized that one has not been involved in the planning of or been explicitly invited to.
He is crashing that trip to get fro-yo, by inviting himself :P
by Matrim Cauthon July 27, 2012