From the second edition (1989):
(ˈsɪnsɪpʌt) Also 7 synciput. [L. sinciput, for early *senciput, f. sēmi half + caput head. So F. sinciput (1586).]

The front part of the head or skull.

1578 Banister Hist. Man 8 The fore part [of the head], called Sinciput, or Bregma. 1650 Bulwer Anthropomet. 11 When the Temples are eminent, the occiput and synciput depressed. 1689 Moyle Abstr. Sea Chirurg. ii. vii, Mind that you apply not your Traphine on the temporal Bones, Sutures, or Sinciput. 1711 Addison Spectator No. 275 ⁋5 We observed a large Antrum or Cavity in the Sinciput. 1767 Gooch Treat. Wounds I. 279 A girl, who‥was struck with the hand upon the sinciput, and became immediately blind. 1809 Byron Bards & Rev. Argt., Wks. 1898 I. 305 Incorporation of the bullets with his sinciput and occiput. a1848 Holmes Nux Postcœnatica 46 All the longest heads That ever knocked their sinciputs in stretching on their beds. 1873 Mivart Elem. Anat. iii. 77 We have the base or basilar region, and opposite to it the vertex, sinciput, or sincipital region.
fig. c1638 Strafford Papers II. 158, I‥judge the other [to be]‥the very sinciput, the vertical point of the whole faction.