The song begins with it's unforgettable bass line and these lyrics:

"Some people call me the space cowboy, yeah
Some call me the gangster of love
Some people call me Maurice
Cause I speak of the pompatus of love"

The first lines are easy. "Space Cowboy" and "Gangster of Love" are old Steve Miller Band songs. Maurice is probably from his song "Enter Maurice". But what in the world does "Pompatus" mean? Look at the lyrics for "Enter Maurice". It contains the line "My dearest darling, come closer to Maurice so I can whisper sweet words of epismetology in your ear and speak to you of the pompitous of love.". Yet another reference to the mysterious P word (spelling varies). And so the mystery remained until "The Straight Dope" got on the case. They tracked down an old song called "The Letter" by a group called the Medallions. It contained a line very similar to the line from "Enter Maurice".
The answer was finally revealed when The Straight Dope tracked down the author of the original Medallions song, Vernon Green. He claims that the orignal word is "puppetutes". He came up with this word to describe his idea woman, a "paper-doll fantasy figure". I guess it's pretty obvious why Steve Miller didn't want to confess the true meaning. He probably had no idea what it meant!