lose one's cool

(idiomatic) To become upset or disconcerted; to lose one's temper.

Example: 1966, "To the Ludicrous," Time, 18 Nov.:
  But in his final singles match against Mandarino, the Menace lost his cool. Visibly rattled by noisy spectators, who chanted "Brasil! Bra-sil!" from the third set onward, he collapsed completely in the fifth set.
2006, Stefanie Cohen, "Naomi Campbell Accused of Attacking Maid in Fit Over Jeans,", 28 Apr. (retrieved 3 June 2009):
  The fiery femme fatale is notorious for losing her cool, and was once accused of hitting her secretary with a telephone and threatening to throw her from a moving car.
When things don't go your way, no matter how trivial, how do you react? Do you lose your cool and explode? [1]