What does "lost" mean here? How does it become "out of someone's mind"?
That's a great question. "Lose it" is actually a common idiom. While you can use it to talk about someone going mad or crazy, it's also often used to mean losing control of your emotions. Ex: I tried to stay calm, but in the end, I lost it. Ex: I was in a meeting last week, totally lost it. Ex: If he asks me to re-type this email one more time, I'm going to lose it.