Neologisms as described by Stedman's Medical Dictionary are meaningless words used by psychotics. Another common term used in psychiatric hospitals is word salad. For example, a person you meet on the street might say to you, "Put your genectagazoink in the farbleflar schlebby noobooshisto!!! Do it NOW before they get you!" Tourrette's Syndrome sufferers will also have recurring neologisms.
Tom Green had an extremely funny and disturbing bit where he was doing one of his man on the street ambush interviews and he started talking to a woman who was clearly psychotic, altered and neologistic. Tom took it upon himself to respond in kind and they had an entire neologistic conversation. It was a classic example of how indulging psychosis only tends to agitate the psychotic even more.
Jerry Lewis made a fortune off of neologisms. The title character of the movie Pootie Tang spoke almost exclusively in a neologistic language. Lewis Carroll also used them extensively in his poem Jabberwocky. He even went so far as to define each nonsense word in the poem with more convoluted neologistic definitions.