lose one's head

To lose one's composure and act emotionally or irrationally. You need to calm down before you talk to Larry. You don't want to lose your head before finding out his side of the story. I'm sorry, I lost my head out there. There's no excuse for what I said.

see under keep one's head, def. 1.

To become so agitated that one cannot act sensibly. This expression, which at one time meant literal decapitation and was used figuratively from the mid-nineteenth century on, differs from the more recent catchphrase “You’d lose your head if it wasn’t screwed on,” addressed to an extremely absentminded person. Thomas Macaulay’s History of England (1855) stated, “He lost his head, almost fainted away on the floor of the House.”