lose one's head
lose one's head (over) {v. phr.}
1. To panic.
"Let's not lose our heads," the captain cried. "We have good lifeboats on this vessel."
2. To become deeply infatuated with someone.
Don't lose your head over Jane; she is already married.
Contrast: KEEP ONE'S HEAD.
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Source: A Dictionary of American Idioms