Noun: Something that exists in the utmost sense of profundity. It is unique, provocative, insightful, and even philosophical; it makes one think deeply.
A book by Albert Camus, "L’étranger" ("The Stranger"), can be characterized as such. The hero of the tale is a sociopathic man, completely out of touch with his emotion, to the extent of arbitrarily killing another person. It is written in such a way that one is led to feel sympathetic with a murderer.
In a sentence:
"I saw Donnie Darko last weekend; the movie was a complete mindfuck."
As applied to philosophy:
"Nietzsche's metaphor in the presence of The Madman and its relevance to the concept of Ubermensch and the notion that humanity has killed God is utterly mindfucking."