[snewman]

Actually the definition of 'grok' is discussed in the book; you can find some relevant snippets at https://en.m.wikipedia.org/wiki/Grok. My recollection is that the book says the original / literal meaning is "drink", but this isn't supported by the Wikipedia quotes and perhaps I am misremembering, it has been a long time.

[NikkiA]

The book also points out that it is much more than just 'drink', and 'drink' would by no means cover 99% of the way it is used in the book.

That said, I've only ever read the full unabridged re-issue from the mid-90s, it's possible the earlier, edited, releases had many of the uses elided.