Nothing. However, a brown bag LUNCH means one that you pack yourself, usually carried in a paper sack.
No. This is not an idiom. An idiom is a group of words whose meaning is different from the meanings of the individual words. So it is not easy to know the meaning of an idiom. For example 'Let the cat out of the bag' is an idiom meaning to tell a secret by mistake. The meaning has nothing to do with cats or bags. "Treat others like you would want them to treat you" is a saying,
"Spilled the beans" is an idiom. There isn't another idiom for it.
"To be" is not an idiom - it's a verb.
The idiom "not your bag" indicates that you're not good at something or you just don't enjoy it. Example sentences: The painting is ugly because art is not your bag. I didn't make the team, but tennis just isn't my bag.
A brown bag
Pest is not an idiom. It's a word.
The idiom "apple shiner" means the teacher's pet.
The meaning of the idiom in the pink of health means being in good health.
The idiom means impress someone is egg on
It's not an idiom - to cope means to deal with, or to handle
"Old hand" is an idiom meaning having lots of experience.