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1.
Crazy, Insane, Bonkers.
Jack: "If we don't find a way out of this soon, I'm gonna loose it."

*Teal'C cocks his head looking confused*

Jack: "Lose it? It means go crazy ... nuts ... insane ... Bonzo ..."

*He's putting more ketchup and mustard on the plate as he's talking*

Jack: "No longer in possession of one's faculties ... Three fries short of a Happy Meal ... WACKO!!!!!!!!!!!!!!!!!!!!!!!!!!!!"

*He shows Teal'C the smiley face he drew on the plate*
by Fishyghost January 18, 2008
 
2.
A slang expression for getting insane.
The "happy meal" means here "insanity", the person is on the edge of being or getting insane. The most popular use of the term, when it wasn't coined by them, was in a episode of the Sci-Fi series Stargate.
O'Neill: Lose it. It means, go crazy... nuts... insane... bonzo... no longer in possessions of one's faculties... three fries short of a Happy Meal... WACKO!!
by borisbaran January 19, 2007