pinnaculum.

From pinna (“plume, wing; parapet”). Pinnāculum appears in the Vulgate as a calque of Ancient Greek πτερύγιον (pterúgion, “pinnacle”), diminutive of πτέρυξ (ptérux, “wing”) (alternative translations of the Greek include fastīgium and pinna itself). Therefore, the end of the word appears to be the neuter form of the Latin diminutive suffix -culus. However, pinnāculum is not a regularly formed diminutive: there is an irregular change of gender from the feminine base and an unexpected -ā- between the base and the suffix. The form may have been influenced by that of nouns ending in -āculum that were derived from the instrument noun suffix -culum: the Oxford English Dictionary suggests that the form pinnāculum was possibly based on analogy with prōpugnāculum (“bulwark, rampart”), from prōpugnō + -culum.