I concede: it took a bit of digging (the shelves in our e-scriptorium* suffer from chronic books-behind-books syndrome), but The New Hacker's Dictionary acknowledges Strangers in a Strange Land as the origin of grok. It's between gritch and grunge.

--- Rats: now my first-edition copy of TNHD is getting even more loose-leaf. Perfect binding isn't.

* For completeness, Herself has a Sulkarium upstairs.