A paradoxal word. It is so because 'yiffniff' cannot actually be spelled, as it was read in the 2007 FCAT writing assessment. It is now referred as a transitive/intransitive verb, an adjective, adverb, an adverb describing a verb/adjective, a noun, or a general exclamation. It is also a great substitute for profanity such as 'fuck', since it is so versatile.
Yiffniff can be spelled (and has been tried to be spelled) as follows:
and so on and so forth.