Postby Baldanders » 2016-04-26, 14:00
No problem. I realize now though, that I failed to consider the times when the phrase takes an adjective, in which case it is clearly a modifier with the sense "very" and not a noun with the adjective omitted as my explanation tried to propose.
If this indeed was its original use, I can't explain how it came about.
It is possible to extract some meaning out of it literally as well. It could mean that someone's quality isn't so great that people would think of him when prompted to name a thing with this quality, so הוא לא מי יודע מה חכם could be (kinda) literally understood as "he wouldn't be anyone's answer for 'what is clever?'" (= "he isn't very clever"), for instance. I actually this is the most intuitive understanding of the phrase, and the answer you would get from someone who won't overthink that as I.