keep (one's) 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

BRITISH, INFORMAL
If someone tells you to keep your hair on, they are telling you in a forceful way to calm down and not be angry or impatient. `All right, keep your hair on,' Ralph said with a shrug. Note: The usual American expression is keep your shirt on.
See also: hair, keep, on
Collins COBUILD Idioms Dictionary, 3rd ed.

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

keep your ˈhair on

(also keep your ˈshirt on less frequent) (British English, informal) used for telling somebody who is angry or very excited about something to keep calm: Keep your hair on, Mum. You can hardly see the damage. Keep your shirt on! We’ve got plenty of time to get to the airport.
See also: hair, keep, on
Farlex Partner Idioms Dictionary
See also:
  • keep your 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!