Frottage is a psychology term for the act that involves getting sexual pleasure from rubbing against something or someone. People who like this are Frotteurs and the whole shebang can also be called Frotteurism.
Folks who really get into this are really hooked on doing this fully-clothed and really like to do this without the other person being aware or complicit. Think of elevator behavior or in crowds in shopping malls or at sports events.
