Solved by @Aedis@lemmy.world https://en.m.wikipedia.org/wiki/Gostak

I seem to remember reading a Wikipedia article about it. The sentence is “The X Ys Z”. The words are gibberish like “Smirkel ghandles bitonipt”. When you ask what is X you answer “X is what Ys Z”, “what does it mean to Y?” “to Y is what X does to Z”.

Am I remembering correctly or is there no such canonical sentence in English?