mid-15c., "heavenly, divine," from Old French supernal "supreme" (12c.), formed from Latin supernus "situated above, that is above; celestial" (from super "above, over;" from PIE root *uper "over") as a contrast to infernal.
It is the hypothetical source of/evidence for its existence is provided by: Sanskrit upari, Avestan upairi "over, above, beyond;" Greek hyper, Latin super "above, over;" Old English ofer "over," German über, Gothic ufaro "over, across;" Gaulish ver-, Old Irish for.
late 14c., "of or pertaining to the underworld," (ancient Tartarus, the sunless abode of the dead, or the Christian Hell), from Old French enfernal, infernal "of Hell, hellish" (12c.), from Late Latin infernalis "of or belonging to the lower regions," from infernus "hell" (Ambrose), in classical Latin "the lower (world)," noun use of infernus "lower, lying beneath, underground, of the lower regions," from infra "below" (see infra-).
Pluto was infernus rex, and Latin inferi meant "the inhabitants of the infernal regions, the dead." Association of the word with fire and heat is via the Christian conception of Hell. Meaning "devilish, hateful" is from early 15c.; meaning "suitable for or appropriate to Hell" is from c. 1600. As a name of Hell, or a word for things which resemble it, the Italian form inferno has been used in English since 1834, via Dante. Related: Infernally.
<a href="https://www.etymonline.com/word/supernal">Etymology of supernal by etymonline</a>
Harper, D. (n.d.). Etymology of supernal. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/supernal