also pleo-, word-forming element meaning "more," from Greek pleiōn "larger, greater in quantity, the more part, very many" (comparative of polys "much"), from PIE *ple- (source also of Latin plere "to fill," plebes, "the populace, the common people;" Greek plēthein "be full," plērēs "full"), possibly a variant of root *pele- (1) "to fill."
word-forming element meaning "before," from Latin prae (adv.) "before," from PIE *prai-, *prei-, from root *per- (1) "forward," hence "in front of, before" (also see pre-). Reduced to pre- in Medieval Latin. According to OED the full form prae- in Modern English appears "usually only in words that are still regarded as Latin, ... or that are terms of classical antiquity ...."
word-forming element meaning "under, beneath; behind; from under; resulting from further division," from Latin preposition sub "under, below, beneath, at the foot of," also "close to, up to, towards;" of time, "within, during;" figuratively "subject to, in the power of;" also "a little, somewhat" (as in sub-horridus "somewhat rough"), from PIE *(s)up- (perhaps representing *ex-upo-), a variant form of the root *upo "under," also "up from under." The Latin word also was used as a prefix and in various combinations.
In Latin assimilated to following -c-, -f-, -g-, -p-, and often -r- and -m-. In Old French the prefix appears in the full Latin form only "in learned adoptions of old Latin compounds" [OED], and in popular use it was represented by sous-, sou-; as in French souvenir from Latin subvenire, souscrire (Old French souzescrire) from subscribere, etc.
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 subcontractor); "inferior" (17c., as in subhuman); "smaller" (18c.); "a part or division of" (c. 1800, as in subcontinent).