(verb) to lose control of your emotions
The poor schoolteacher blew her cool and started crying when she won five million dollars in the lottery.
The workers complained to their boss because the manager was always blowing his cool over unimportant things.
Boyfriend: We need to talk.
Boyfriend: Please don’t blow your cool, but I think we should date other people.
Girlfriend: What?! I thought we were going to get married! I hate you!
Boyfriend: I asked you not to blow your cool!