It is an order a parent or teacher might give to a child. it means grow up (behave like an adult), or stop being lazy.

a smilar idiom would be "pull your socks up".

It is not something we say often. You would need to tell us more about what was happening when you heard it. My best guess would be, either what Melissa said or it could mean to grow up.

I think it literally means to pull your pants up like you are wearing your pants?