I googled it, and here it is in the Urban Dictionary.
incapable of being understood, failing to be understood correctly.
I LOVE some of the answers. What a cool sight. The English language continues to evolve. Every year new grammar rules are created. To this board of Deciders I would suggest that if a word is google-able, then it should be able to be published. I guess it is ultimately the author's choice, depending on what is being writ. I am also of the kind of wordsmith [get-sic] that Likes to makeUp rules and Words.
"Being brain injured, the challenges we face are inexplicable, or at least, ununderstandable."
blog entry essay with "UNUNDERSTANDABLE"