a prefix occurring originally in loanwords from Latin (subject; subtract; subvert; subsidy); on this model, freely attached to elements of any origin and used with the meaning “under,” “below,” “beneath” (subalpine; substratum), “slightly,” “imperfectly,” “nearly” (subcolumnar; subtropical), “secondary,” “subordinate” (subcommittee; subplot).
a prefix indicating a basic compound: subacetate; subcarbonate; subnitrate.
a prefix indicating that the element is present in a relatively small proportion, i.e., in a low oxidation state: subchloride; suboxide.
word-forming element meaning "under," from Latin preposition sub "under" (also "close to, up to, towards"), from a variant form (*(s)up-, perhaps representing *ex-upo-) of PIE root *upo- "from below," hence "turning upward, upward, up, up from under, over, beyond" (cf. Sanskrit upa "near, under, up to, on," Greek hypo "under," Gothic iup, Old Norse, Old English upp "up, upward," Hittite up-zi "rises"). Used as a prefix and in various combinations.
The original meaning is now obscured in many words from Latin ( suggest, suspect, subject, etc.). The prefix is active in Modern English, sometimes meaning "subordinate" (as in subcontinent, first recorded 1863) or "inferior" (a sense first attested 1963).