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3.
Used by stoners when they find something funny, usually said with no emotion.
Vanessa: Did you hear about what happened to Todd?
Ben: Crack up.
by Ark T May 12, 2010
 
1.
Something very funny.
When talkin' bout Coke.
"Hahaha, whatta crack up!"
"Yo, Crack up, foo'."
by Diego September 04, 2003
 
2.
To burst into loud, hysterical or aggressive verbal abuse. Used primarily in Ireland and the UK. Similar with the American "freak the fuck out," however only used in negative situations.
"Yer wan at the pub cracked up at me when I asked how much she charged for an hour."

"Don't you dare crack up at me! I'm not the one sleepin' with my sister!"
by Lolitaslut May 06, 2009
 
4.
A female pilot flying upside down. An FAA rating obtained by female pilots after they have logged a minimum of 15 hours of inverted flight time.
Sally is a crack up. She can fly better upside down than most pilots can fly right-side up.
by seymour heines February 28, 2008