noun. Along with God-mentum and dog-mentum, one of the strongest forces in the greater metropolitan region.
Real citation: "The next two types of mentum that we must be aware of produce the best results when they are as equally balanced as possible. One is called buttmentum. Buttmentum is the theory of the downward force produced against the human body that is in direct proportion to the mass of the human's butt. People with large butts must contend with a stronger force of buttmentum than those who have smaller butts."
(Dec. 12, 2007, Grandma is an Idiot!, http://grandmaisanidiot.blogspot.com/2007/12/mentum.html)
Made-up citation: "After six years of working out, I have honed my buttocks to a razor-sharp edge. I am in awe of my ungodly buttmentum, as are the villagers. I can open beer bottles, soothe angry mobs, and perform invasive surgery with this thing."