Heinlein in the book Stranger in a Strange Land. and iirc (it's been decades) it was used in the context of a "cult" - the people in the cult "grokked" and the people who weren't didn't.

I comment something like this when i see the word grok, which has happened more this year than in the last decade - and i am not sure why, probably because i don't pay much attention to OAI news?