Wiktionary
lose her cool
vb. (alternative form of lose one's cool English)
Usage examples of "lose her cool".
She was trying hard to stifle her fury, for the sake of her daughters, who'd never seen their mother lose her cool.
She only hoped it was not delayed, because she was coming to the point, with Momma and the Grove, and the job at the Steak House, when she would lose her cool completely.
He realized, too late, that the worst embarrassment for someone Louise's age was to lose her cool, to fail to cope.