To lose one's composure. Most people lose their cool when prompted by a negative impetus, like sexual rejection or poor brunch service.
"Losing one's cool is the sexual equivilent of an angry, limp penis."
by Poop.butt. April 10, 2014
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Not staying cool and in control. The opposite is to keep one's cool.
Don't lose your cool, man!

One will fail if one lose one's cool.
by jimmybone d May 27, 2008
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