sidus.

Tentatively thought to derive from a Proto-Italic *sīþos (“mark, target”), with a putative semantic shift (from hunters' jargon) from "target" > "star". Further origin unclear, with multiple theories proposed: * Some derive this from Proto-Indo-European *sweyd- (“sweat”), whence Latin sūdor, Ancient Greek ἱδρώς (hidrṓs), English sweat. * Rix, connecting the word to Ancient Greek ῑ̓θῠ́ς (īthŭ́s, “straight, just”) and Sanskrit साध् (sādh, “to go straight to a goal”), derives the terms from a Proto-Indo-European *seHdʰ- (“to achieve a goal”), and the Latin from a reduplicative form *si-sHdʰ-u- > *sisdʰu- > *sizdu- > *sīdu-. This is semantically tenuous. * Compare Ancient Greek σίδηρος (sídēros, “iron”). This is semantically unconvincing. * Possibly borrowed from a substrate. * Perhaps from a Proto-Indo-European *sh₂i-dʰh₁-o- (“binding”), from *sh₂ey- (“to bind, tie, fasten”), whence saeculum (“age, race, generation”). De Vaan tentatively prefers this derivation.