Lose one's cool Informal

To suddenly become angry, upset, or lose one’s composure, especially after a period of self-control.

It’s typically used to describe a visible loss of control, whether through shouting, arguing, or otherwise breaking from one’s usual calm behavior.

The idiom rests on “cool” used as a noun meaning composure or self-control, a usage that historians of slang trace to mid-20th-century American jazz culture. “Cool” had long existed as an adjective meaning calm or detached, but its rise as a noun describing an admired, unflappable temperament is closely tied to the “cool jazz” style epitomized by Miles Davis’s 1949 recordings, later compiled as the album “Birth of the Cool,” which deliberately contrasted its restrained sound with the more intense “hot jazz” of earlier decades. From this jazz-scene sense of “cool” as a prized emotional steadiness, slang expressions like “keep your cool,” “blow your cool,” and “lose your cool” developed through the 1950s and 60s counterculture; the Oxford English Dictionary’s earliest citation for “lose one’s cool” in this sense dates to 1964. The phrase’s relatively recent, clearly traceable origin in a specific mid-century American subculture makes it unusual among idioms, most of which have murkier or disputed beginnings.

He finally lost his cool after the third delayed flight in a row.

Try not to lose your cool during the interview, no matter how tough the questions get.

The coach lost his cool on the sideline and got a warning from the referee.

A: Did she seem upset about the criticism? B: Upset? She completely lost her cool.