Used when someone can't contain their anger, frustration, or excitement and they express it strongly. When he lost the game, he lost it and yelled at everyone.
Pro Tip 2/3
Colloquial Speech
Common in casual conversation, not suitable for formal contexts like business meetings or academic writing. At the party, when the music stopped, Maria lost it and got everyone laughing with her reaction.
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Sudden Change
Implies a quick or unexpected shift in someone's composure or emotional state. He was fine until he read the news and totally lost it.