About the phrase " pick someone brain"

The phrase should be "pick someone's brain," and suggests interrogating them about a particular topic in order to learn more about said topic.

I think it is used often enough that practically all native speakers are familiar with it.

Often, a person will draw someone aside, ask "Can I pick your brain?", describe a situation, and then say "so what do you think?"

If you are picking nutmeats out of a walnut, you jab and dig at the shell to get at what's hard to get at. Likewise, picking someone's brain is intended to get insights that are hidden, although without the trauma a nutpick might inflict.

It's often not an educated "expert" pro se, but someone with experience and common sense, and instead of answers to specific questions, it's their judgment that is wanted.

Most Americans would have encountered this idiom by the time they are teenagers. although as a teen, one knows everything and has little need to pick someone's brain.

According to the Free Dictonary:

to talk with someone to find out information about something.

And gives the following example:

I spent the afternoon with Donna, picking her brain for ideas to use in our celebration. Do you mind if I pick your brains? I need some fresh ideas.

But if the context is a zombie party the meaning can be different. :-)