The result of a man in tight, khaki shorts sitting on a staircase with his legs spread wide open, so that the testicles are pressed tightly against the seam, creating an ass-like shape. Can also be used in replacing a noun.
That guy on "Trading Spaces" has buttnuts!
Hey, buttnuts, get over here!
Another word for hemorrhoids.
Damn, my buttnuts are really burning!