keep one's hair on

Verb

keep one's hair on (third-person singular simple present keeps one's hair on, present participle keeping one's hair on, simple past and past participle kept one's hair on)

1. (Ireland, UK, colloquial, idiomatic) To stay calm; to be patient; to calm down.
   • All right, all right, just keep your hair on, mate.
   • “Oh, nothing, nothing!”
     “Yes, there’s something. I mean to know!”
     “Keep your hair on; it’s nothing.”
     “Out with it!” cried Addie, scarlet with rage.
     And he flew at Jaap’s throat.1901, Louis Couperus, translated by Alexander Teixeira de Mattos, Small Souls

   Synonyms:

   keep one's knickers on, keep one's pants on, keep one's shirt on, keep one's wig on