Keep your shirt on

[shawnmmurphy]

Where did the phrase 'keep your shirt on' originate from?

[Quizmonster]

From the mid-19th century in the USA, 'to get someone's shirt out' meant to make him annoyed. Perhaps it was based on some sort of schoolboy trick, whereby one lad would actually pull another's carefully-tucked-in shirt out of his trousers.

Another possibility is that - since taking one's shirt off is often a preliminary to a fist-fight (or used to be) - telling someone to 'keep his shirt on' is no more than a suggestion to calm down and forget the fight.