The name seems to be a sort of joke. The Beatles had released the film "HELP", and the physical beginning of a movie is called the "HEAD". "Head" is actually written on the beginnings of every reel of film produced just as "Tail" is written on the ends. A similar joke can be seen later in the movie - when they are playing a concert, you can see the word "Drum" on their drums instead of "Monkees". Additionally, a "head trip" was a common term in the late sixties, and this can also be part of the joke. In 60s and 70s slang, "head" meant someone who used psychedelic drugs, as in "pot head," and "acid head." The name is also suggestive of oral sex.
During production, the working title for the film was "Changes", which was later the name of an unrelated album by the Monkees. A rough cut of the film was previewed for audiences in Los Angeles in the summer of '68 under the name of "Movee Untitled".
The film featured Victor Mature as "The Big Victor" and other cameo appearances by Nicholson, Teri Garr, Carol Doda, Annette Funicello, Frank Zappa, Sonny Liston, Timothy Carey and Ray Nitschke. Wikipedia