"Pfister" is likely another piece of adult humour by Rockstar, meaning "fister" as in someone who sticks their fist into the female genitalia or male or female anus.
The name could be a reference to Pfizer, a brand who produce the viagra medics in 1999.
In GTA San Andreas, pedestrians in areas like Paradiso and San Fierro can be heard saying what sounds like, "Have you driven a Pfister?" Sometimes, if the player is driving an expensive-looking vehicle, a pedestrian might comment positively, "Is that a Pfister?" This could mean the brand was planned to be introduced before GTA IV with the scrapped system of vehicle brands (as seen in carcols.dat).