köken: From Proto-Italic *noktowe, from Proto-Indo-European *nokʷt-ew (“by night”). Cognate with Proto-Tocharian *nekʷtu- (“by night”) and Sanskrit अक्तु (aktú, “by night”). The noun form is either from *noctūd/*noktou, ablative and locative singular of *noktus or rebracketed from the adverb; compare Sanskrit अक्तु (aktú, “night”) and अक्तु (aktú, “by night”).
Noctuque et diu ut viro subdola sis,
[…] commune alacres […] continuum diu noctuque iter properabant.
köken: Uncertain. Maybe from Proto-Indo-Aryan *aktúṣ, from Proto-Indo-Iranian *aktúš, an ablaut variant of Proto-Indo-European *nokʷt-ú-s (“night”); compare, though with potential phonetic and semantic issues, Ancient Greek ἀκτίς (aktís, “ray, beam”). Both the Greek and Sanskrit words are traditionally assigned to a root *h₂ek-t- (“ray, beam”), but this fails to explain the “night” and related senses…