An Etymological Dictionary of Astronomy and Astrophysics

A prefix occurring originally in loanwords from L. but freely attached to elements of any origin and used with the
meaning “under, below, beneath.”

Etymology (EN): From L. preposition sub “under” (also “next to, up to, toward”), from PIE base *upo- “from below,” hence “turning upward, upward, up, over, beyond;” cf. O.Pers. upā (prep.) “under, with;” Av. upā, upa (prep.; prevb) “toward, with, on, in” (upā.gam- “to arrive at,” upāpa- “living in the water,” upa.naxturušu “bordering on the night”); Mod.Pers. bâ “with,” from abâ; Skt. úpa (adv., prevb., prep.) “toward, with, under, on;” Gk. hypo “under;” Goth. iup,
O.N., O.E. upp “up, upward.”

Etymology (PE): Zir- “below, down;” Mid.Pers. azêr “below, under,” êr “below, down; low, under,” adar “low;”
Av. aδara- (adj.), aδairi- (prep.) “below;” cf. Skt. ádhara- “lower;” L. infra (adv., prep.) “below, underneath, beneath,” inferus “lower;” O.E. under “under, among”);
PIE base *ndher.