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lose (one's) cool
To lose control of one's composure, temper, or nerve in a given situation. He really lost his cool when the waiter dropped his food. I really lost my cool during the interview.
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.
lose one's cooland blow one's cool
to lose one's temper; to lose one's nerve. Wow, he really lost his cool! What a tantrum! Whatever you do, don't blow your cool.
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs. © 2002 by The McGraw-Hill Companies, Inc.