Back in the days of the first hippies, uptights (Straights, drones) added "Dirty" in front of "hippie" to try further degredate Free Thinkers. Very simply, the straights started calling Free Thinkers "hippies" and it dinna bother them. So they started calling them "Dirty hippies" to try to get a raise out of them.
That guy asked me if I had a sister who wanted free love. The dirty hippie.