keep one’s pants on

(idiom) be patient

A: Are you coming? We’re going to be late!
B: Keep your pants on! I’m almost ready!

Wilma told her husband Fred to keep his pants on when they were on their way to the drive-in movie.

From the Blogs:

Keep your pants on: The Birth of an Internet Meme

Media:

Similar idioms:

1. ants in one’s pants
2. caught with your pants down