Keep One's Hair On Meaning
(idiomatic, Britain, colloquial) To stay calm; to be patient.
Example: All right, all right, just keep your hair on, mate.
1901, Louis Couperus, Alexander Teixeira de Mattos (translator), Small Souls,
â€œOh, nothing, nothing!â€
â€œYes, thereâ€™s something. I mean to know!â€
â€œKeep your hair on; itâ€™s nothing.â€
â€œOut with it!â€ cried Addie, scarlet with rage.
And he flew at Jaapâ€™s throat.