1.One who's level of intelligence is comparable to that of their own ass.
2.One who's actions suggest that they might be thinking with their own ass, though it, like their brain, lacks the ability to make intelligent decisions.
See also dumnass
(Don't spend too much time on these alternatives, sperm-for-brains. They all mean the same thing.
Maybe if you weren't such a dumass, people wouldn't kick you.