From the second edition (1989):
(ˈneɪtɪfɔːm) [f. L. nat-is (see nates) + -(i)form.]
Resembling or having the form of buttocks.
1681 tr. Willis' Rem. Med. Wks. Vocab., Natiform, in the form of a buttock. 1839–47 Todd's Cycl. Anat. III. 384/1 The natiform protuberances are unusually large. 1898 Dawson Williams Med. Dis. Inf. 262 The skull assumes a peculiar and characteristic shape, to which the term natiform has been applied.