@previous (Catherine !TGirlYJKXM)
From my understanding it ends up which is used most often and IF ENOUGH then the Oxford Dictionary people make a decision on If OR NOT to include in next addition.
@previous (Big Daddy Derek !Uvm54ORbmo)
Intelligence is something you will never have.
@OP
I think etiquette would dictate not using "fucknugget" or "fuck nugget" in conversation at all. Since the phrase in question is slang and has no officially recognized form, whether you make it one or two words is purely a matter of personal style at this point. If you must use it, I would encourage you to use whatever form you feel most effectively conveys your meaning.
(Edited 1 minute later.)