Butt-shaped, yeah. No, really. Ammon Shea reports in Reading the OED that he was surprised to discover this has never been used as an insult, at least according to the OED's citations -- and I am too. I mean, it's perfect for one. First attested in 1681 (coined from Latin roots nates, plural of natis, a buttock + -form, shape) but apparently never used outside of medical contexts -- and even there, it has only survived as "natiform skull," bony nodules on the surface of the skull in infants with congenital syphilis, and even those are now more commonly called Parrot nodes. So twelve-year-olds of the Internet, your task is clear -- revive this word!
His natiform chin was the outward sign of his inner buttheadedness.