Uncertain origin, but possibly from Proto-Indo-European *negʷ- (“bare, naked”) if this root is assumed also to be the source of *nókʷts (“night”) (Latin nox), thus “black” would attest the intermediate meaning between “bare” and “night”. Possibly cognate with Umbrian niru (“(possibly) dark, black”), though this form (thought to be accusative singular) is not mentioned in De Vaan.