This 表現 is actually a 表現, and not a ことわざ. I came across it for the first time well over a year ago, in an article that Claytonian of The Hopeless Romantic sent me. The article is about a hidden restaurant that a guy runs out of his junk/memorabilia filled house somewhere in Nagasaki, and though I could understand a fair amount of it back then, there was one phrase that was too far over my head, no matter how hard I tried to figure it out.
しる ひと ぞ しる
shiru hito zo shiru
Only those who know, know, and they know it well.
Conversely, if you're not in the know, you're not.
The hidden restaurant in the article remains hidden, because it refuses to divulge its address, referring to it only as 知る人ぞ知る長崎の穴場スポット: A nice little spot in Nagasaki, well-known and well-liked by a select few. Both Clay and I were of the opinion that it would be really, really cool to figure out a way to become 常連 or お得意 さん at a place like that.
There were a few reasons I couldn't figure this phrase out at the time: context, timing, and a lack of grammar knowledge.
When I got a Japanese friend to help me try to find out where this place was, they explained 知る人ぞ知る, by saying "People who don't know, won't know," which is a decent job of translating contextually, but not literally. Literally, this expression emphasizes KNOWING, but her translation put the emphasis on NOT KNOWING. Which was bad timing because I had just learned the ～ずに construction, I kept trying to think up a way to connect 知る人ぞ, to the idea of negation, like you would do with 知らずに. Why? Because they both had 'z' sounds, and because I wasn't yet familiar with the way to use こそ to stress a subject.
Turns out that ぞ, is yet another archaic feature of Japanese, still preserved in some turns of speech today. It's an old way of saying "こそ," which makes it much, much easier to understand. Who knows? People who know こそ know.
Why is it that even though you're a foreigner, you know more yojijukugo than your average Japanese person?
It's thanks to a certain site that those of us in the know use...