Hot answers tagged fair-use
First off, "grok" is not copyrighted; you can't copyright individual words, even made-up ones. Therefore fair use (a defense against an infringement claim) does not apply. That doesn't mean it's impermissible, in fact it almost certainly is fine. It's also not trademarked, as it is not being used by the Heinlein estate to identify a product or service. And ...
Kindall tackled the legal aspect. As for reception/perception considerations, here's the rule of thumb I'd use: If you're using the same word in the same way for the same thing, and your story is about that thing (or concept, or whatever) - you're crossing the line. That's like saying "I'm writing a story about the same Smeerps Albert J. Jones wrote about," ...
Actually you can prevent others using your made up words by having it registered as a trademark. In your case, you can cite fair use especially since the word is now common and known outside of the original author's creation. The word 'grok' as was pointed up above is part of the common English language now and it would be difficult to find grounds to ...
IANAL, but I don't think they can sue you (successfully) for using a word which is part of the (informal) English language: http://www.merriam-webster.com/dictionary/grok http://oxforddictionaries.com/definition/grok Even my favourite dictionary lists it.
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