bottom-joust
The two (or more) participants bend over with their bottoms facing inwards, they then run together and their bottoms collide. Generally there are no winners although sometimes extra rules are added depending on the game situation. e.g. whoever falls over first loses
I bottom-jousted with my friend earlier today.
A stranger beat me at bottom-jousting on my way to work.
A stranger beat me at bottom-jousting on my way to work.