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Open ~/Library/Spelling/LocalDictionary in TextEdit. Remove the word in question, save. It is possible that there could be subfolders if you work with different languages.  
 
Open ~/Library/Spelling/LocalDictionary in TextEdit. Remove the word in question, save. It is possible that there could be subfolders if you work with different languages.  

I can get to ~/Library/Spelling but there is nothing in the spelling folder. No LocalDictionary, it's just empty.
 
Be sure you are going to your Library folder. The ~/ denotes your home directory.
 
Instead of doing things the hard way, you could always just do Spelling -> Spelling... and click on "Forget" for the word in question.

--Eric
 
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