"deep, unnatural sleep; lethargy," 1670s; earlier in a figurative sense (1650s), from Latin sopor "deep sleep," from suffixed form of PIE root *swep- "to sleep," the source also of Latin somnus, Greek hypnos.
Entries linking to sopor
*swep-
Proto-Indo-European root meaning "to sleep."
It forms all or part of: hypno-; hypnosis; hypnotic; hypnotism; insomnia; somni-; somnambulate; somniloquy; somnolence; somnolent; Somnus; sopor; soporific.
It is the hypothetical source of/evidence for its existence is provided by: Sanskrit svapnah, Avestan kvafna-, Greek hypnos, Latin somnus, 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."