New vocab term

I learned a new word this week. My fifth graders were lining up to leave at the end of the day. Two bent over to pick up their backpacks and when they were coming up they (not sure how it could really be done accidentally) but they "lip butted." What in the world is that? you ask. That is what they said in surprise when they stood back up, anyway. I guess lip butting is sorta like head butting, but with your lips? The two boys didn't really seem to phased by the fact that they just kissed in my line, I guess it is manly when you call it lip butting. Maybe it isn't a useful term, but it is amusing.