Keep your shirt on!
idiom

- a slightly impolite way of telling someone who is angry to try to be calm and patient.

Cambridge International Dictionary of Idioms

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WORD ORIGIN

In the western film classic Big Country, tough guy Charlton Heston challenges city gentleman Gregory Peck to a fight in the middle of the prairie. Before they start, Heston takes off his shirt to reveal his muscles. Peck, being the refined man that he is, keeps his shirt on. This is a good illustration of where we get the expression "keep your shirt on." Heston is the one who is angry and can't wait to fight. Peck on the other hand is calm and cool.

Figuratively speaking, there are several other things you can do with a shirt:

- Lose your shirt/Put your shirt on something - to risk all your money on something because you are sure you will win

- Would give you the shirt off their back - if someone would give you the shirt off their back, they are extremely generous

- A stuffed shirt - someone, especially a man, who behaves in a formal, old-fashioned way and thinks they are very important

- A hair shirt - if someone wears a hair shirt, they choose to make their life unpleasant by not having or experiencing anything that gives them pleasure

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SYNONYMS

Chill out! Take it easy! Stay cool! Hold your horses! Relax!

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Practice OWAD in a conversation:

"KEEP YOUR SHIRT ON, will you? I'll have the report finished in 10 minutes, OK?"