Frotteurism is a paraphilia where the frotteur derives sexual pleasure from rubbing (usually their penis) on a non-consenting person.
This is a really rude paraphilia. Actually it's a criminal offence. As is ANY NON-CONSENSUAL SEX ACT.
Acting on frotteuristic desires could get you arrested or sent to a psychiatrist. Frotteuristic Disorder is listed in the DSM-V.
The prevalence of frotteurism is debated. However if you're a girl who's been on the London Underground in rush hour, you might predict it is very prevalent.