The folks at Sensual Arousal ask the question . . . have you ever heard of ‘tribbing’? (very NSFW) . . . well, it’s a short for tribadism after the old term tribade, lesbian, and describes the practice of rubbing female genetalia against each other. While the word isn’t very commonly used, there are still a number of sites dedicated to it (most of them are aimed at guys – after all, according to most surveys, the number one male fantasy is the lesbian or bisexual woman fantasy – but there are a number of instructional sites and erotica pieces related to tribadism that are aimed at women as well). It has nothing to do with tribbling or tribbles or Star Trek. It is also known by a number of slang expressions such as scissoring, bumping monkeys or the like . . . one Chinese term is mó jìng (polishing mirrors). See the entry at Wikipedia for more than you want to know on the subject.