"sleep personified; the god of sleep in Roman mythology," equivalent of Greek Hypnos, son of Night and brother of Death, 1590s, from Latin somnus "sleep, drowsiness," from PIE *swep-no-, from root *swep- (1) "to sleep" (cognates: Sanskrit svapnah, Avestan kvafna-, Greek hypnos, Lithuanian sapnas, Old Church Slavonic sunu, Old Irish suan, Welsh hun "sleep," Latin sopor "a deep sleep," Old English swefn, Old Norse svefn "a dream").