oculus.

Etymology tree Proto-Indo-European *h₃ekʷ-der. Proto-Indo-European *-lós Proto-Indo-European *-elós Proto-Italic *-elos Latin -ulus Latin oculus De Vaan reconstructs Proto-Italic *okʷ(e)los m or *okʷ(e)lom n, ultimately from an uncertain descendant of the Proto-Indo-European root *h₃ekʷ- (“to see; eye”) suffixed with -ulus (from Proto-Italic *-elos, from Proto-Indo-European *-lós). Witczak 2021 argues it was originally a neuter diminutive, whereas Sen 2015 sees it as an agent noun formation. Compare the gender of the cognates Sanskrit अक्षि n (ákṣi, “eye”) and Ancient Greek ὄσσε n (ósse, “eyes”) and ὤψ f (ṓps, “eye”). If the noun was originally neuter, then the masculine nominative singular oculus may be a back-formation from an original neuter dual form oculī (reinterpreted as a masculine nominative plural), as proposed by Alan Nussbaum. For other survivals of the second-declension neuter dual ending, see also the masculine-like plurals of caelum (“sky”), frēnum (“bridle”), and rāstrum (“rake”).