It may predate Agent X himself, Stilly: "grok" is in my first-edition copy of The New Hacker's Dictionary, and in its source document, the Jargon File*, which goes back in various guises to the 1960s. I decline to comment further on Agent X, as I've more respect for my digestion.

* I used to promote that at work as "How to understand your friendly neighbourhood MaJoC".