Grok

The term originated in Robert Heinlein's novel Stranger in a Strange Land, where it is used by a man raised with Martians and literally means "to drink".

A character in the novel (not the primary user) defines it:

"'Grok' means to understand so thoroughly that the observer becomes a part of the observed -- to merge, blend, intermarry, lose identity in group experience. It means almost everything that we mean by religion, philosophy, and science -- and it means as little to us (because we are from Earth) as color means to a blind man."