This is a really useful phrase in spoken English.  Everyone does this from time to time.  If you “lose it” it means that you got really angry or “lost control” of your emotions.  This means almost the same thing as losing your temper.  “Losing it” is more commonly used than “losing temper”

Here is a sentence that could be used in real life:

The boss must be really stressed out lately.  He’s usually a really calm guy.  A couple days ago some girl was only 3 minutes late for a business meeting and the boss totally lost it and then fired her.

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