A word created and used by Robert Heinlein in his book 'Stranger in a Strange Land'. IIRC (and it is decades since I read it) in that book it means to know and understand something deeply and completely, but even more deeply and completely than that description implies.
So it is used as a shorthand for 'do you get it?'
(A wonderful book, in the fullest sense of the word - to be wondered at, in many ways).
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