Something natiform resembles or has the form of the buttocks. It derives from Latin nates, plural of natis, a buttock.
Ammon Shea mentions this word in his book Reading the OED, in which he records his experience of spending a year scanning the second edition of the Oxford English Dictionary cover to cover, all 21,730 pages of it. He wrote that he was surprised to learn that it had never been used as an insult, so refuting the premise of an entry in Depraved and Insulting English, which he earlier wrote with Peter Novobatzky.
This may be explained easily enough. The word never moved beyond a limited medical circulation and so it never gained the instant recognition necessary for it to be applied insultingly. Also, it has never been employed to refer to the buttocks themselves, instead always to some anatomical feature that contains a deep cleft.
The OED marks it as obsolete, though natiform skull, bony nodules on the surface of the skull in infants with congenital syphilis (also called Parrot nodes), is in some current medical dictionaries.