Perhaps it's just been by a series of improbable coincidences that in the years I have read forums, IRC logs, books, and articles, I've only seen the term here on HN.
Could be a case of Baader-Meinhof [1], or maybe you haven't been socializing in right forums. The word "grok" has been in popular use on Usenet since the early 1990s at least, probably before, since it's from a 1970s Heinlein novel.
"To really understand something REALLY well".