Remain calm and patient; don't get excited or angry. It's used to tell someone to control their temper or not rush into action.
This American phrase dates to the mid-1800s and refers to the literal act of keeping one's shirt on during a fight. In that era, men would remove their shirts before engaging in fisticuffs to avoid tearing their clothing, so keeping your shirt on meant staying out of the fight entirely.
The phrase captures a fascinating slice of 19th-century masculine culture where shirt removal was such a standard pre-fight ritual that NOT doing it became synonymous with avoiding conflict altogether. It's one of the few idioms where the literal action (keeping clothing on) directly mirrors the metaphorical meaning (staying calm).
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