Keep your hair on
Definition 1 of 1
Pro Tip 1/3Colloquial Usage
Used in casual conversation, not in formal situations like job interviews or official emails. 
I know you're stressed about the results, but keep your hair on; they'll be announced soon.
Pro Tip 2/3British English
It's a British phrase, so non-British English speakers may not be familiar with it. 
When I told my American friend to 'keep your hair on,' she looked confused.
Pro Tip 3/3Non-literal Meaning
It doesn't concern hair; it's a metaphor for telling someone to relax. 
Hey, keep your hair on, the taxi's just around the corner.