According to the Oracle Google:
The word pompatus, also spelled pompitous, //,
//, is a nonce word used in the lyrics of Steve Miller's
1973 rock song "The Joker".
Well, that helps a bit, but..............................................
A nonce word is a lexeme created for a single occasion to solve an immediate problem of communication. The term is used because such a word is created "for the nonce" and is thus "an invented or accidental linguistic form, used only once". All nonce words are also neologisms (newly created words that have not entered the lexicon of a language). Some nonce words have a meaning and may (or may not) become an established part of the language, while others are essentially meaningless and disposable and are useful for exactly that reason, for instance in child language testing.  The term nonce word was apparently the creation of James Murray, the influential editor of the Oxford English Dictionary.
Well, that helps a lot.