Uncertain. Ultimately from Proto-Indo-European *sweyd- (“to sweat, perspire”). It is perhaps from Proto-Indo-European *sweydōs, though this would require a development from *swei- to *swoi-. Alternatively, it may derive from the introduction of the o-grade *swoi- into an s-stem term. Cognates include Ancient Greek ἱδρώς (hidrṓs), Sanskrit स्वेदते (svedate) and Old English swāt (English sweat).