To rub your scrotum on something, as seen by the scene in the movie "Stepbrothers" starring Will Ferrell and John C. Reily, where Will Ferrell's character rubs his ballsack on John's drumset.
Juan: Who's this Harry geezer?
Tom: Oh he's my sisters boyfriend, that makes him my stepbrother-in-law Juan: Surely he's your stepbrother?
Tom: Nah fam, he's not married to my sister