"Frottage" is the act in which one afflicted with "Frotteurism" engages. Does that help?
Seriously, it's basically rubbing one's genitals against an unsuspecting person, usually in a public place.
For example, the NYC subways are full of frotteurs.
Imagine: Here you are, TSW, just strap-hanging to the next stop, unaware of what's happening, and there's a gay man behind you rubbing his erection on your buttocks, but you perceive it as simply a crowded subway car where people bump into one another.
Are you now realizing you've been frotted before? :idea: Isn't it gross?