Underman, or undermensch, is the opposite of Nietzsche's 'Uberman', or overman. Underman is what man will devolve into, and will assume a state close to that of a feces-flinging chimp.
John: Did you see the Henderson kid? He can't do anything but drool and suck applesauce.
Jack: This is what happens when paternal twins marry: they produce the underman.