Yes, informal; I wouldn't call it slang. Why don't you google for a range of examples in your estimation and then post them here?
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MUSCOVITEOK. Let me 'narrow down' my query then.(1) Based on my googling, it looks like this expression has two variants, 'pick somebody's brain' (1) and 'pick somebody's brainS' (2).Btw, my Longman only mentions (2). Both versions are equally common/correct English?(2) Following is an excerp from
Mister Micawber this expression is (thought of by some people as) TOO informal?-- No, not a at all.But MM, Oxford's says it is informal. Is it not really so?
MUSCOVITEI’d like to pick your brain.Really? Does that even sound nice? Your mother might tell you that’s something nice people don’t say at the dinner table.Don't let this excerpt confuse you. The author is making an implicit comparison between "pick your nose" and "pick your brain" purely on the similarity of the grammar, and purely for comic effect. Picki
Mister MicawberBut MM, Oxford's says it is informal. Is it not really so?-- I also said it was informal, but it is not 'too informal'—there is nothing rude or slangy about the phrase...which I use frequently.Thank you very much.