Don’t get your knickers in a twist
The expression “don’t get your knickers in a twist” is something you say to a person when you want them to calm down. When a person is very afraid of doing something, and they are acting very scared and worried, they become jumpy.
Being jumpy means that their underwear (knickers) will get all twisted up.
This is a very old expression, but it’s still used today, and it still means to calm down. If a person stops being jumpy they won’t get their knickers in a twist.
A direct synonym to the expression is