keep your hair on!

keep (one's) hair on

To remain patient and calm, especially when beginning to become angry or upset with impatience. Usually said as an imperative. Primarily heard in UK. Yeah, yeah, keep your hair on! I'll have the report finished by the end of the day! A: "That lady is pestering us about getting her car back today." B: "We need another day to get it fixed. Tell her to keep her hair on!"
See also: hair, keep, on
Farlex Dictionary of Idioms.

keep your hair on!

used to urge someone not to panic or lose their temper. British informal
See also: hair, keep
Farlex Partner Idioms Dictionary
See also:
  • keep (one's) hair on
  • keep your hair on
  • keep (one's) socks on
  • chill out
  • keep at
  • keep at (someone or something)
  • get ants in the/(one's) pants
  • naff off
  • cheer up
  • clear the way