Keep your shirt on
Definition 1 of 1
Pro Tip 1/3Stay Cool
Used when someone is overreacting or getting too excited about something minor. 
They'll announce the results soon. Just keep your shirt on!
Pro Tip 2/3Casual Tone
The phrase has a casual, informal tone and might be inappropriate in formal situations. 
You don't need to yell; keep your shirt on, we're handling it.
Pro Tip 3/3Not Literal
It’s a metaphorical expression; it doesn't mean to physically keep your shirt on. 
The bus is late again? Well, keep your shirt on, it'll come eventually.