What do you dress up for the idiom can it?

The idiom "can it" doesn't relate to dressing up. It means to stop talking or doing something, often in a rude or impatient way.

For example:

* "Can it, I'm trying to concentrate!"

* "Can it with the gossip already!"

There's no specific dress code associated with this idiom. It's a command to stop doing something, not a situation where you would dress up.