word-forming element meaning "old," from Greek presby-, combining form of presbys "elderly, aged," as a noun, "elder, old man," which is of uncertain and much debated origin and phonetic development (some Doric forms have -g- in place of -b-). The first element is likely *pres- "before, in front" (from PIE root *per- (1) "forward," hence "in front of, before, first"). Perhaps the whole originally meant "one who leads the cattle," from the root of bous "cow." Watkins, however, has it from PIE *pres-gwu- "going before," with second element from root *gw-u- "going," a suffixed form of root *gwa- "to come."
word-forming element usually meaning "with, together," from Latin com, archaic form of classical Latin cum "together, together with, in combination," from PIE *kom- "beside, near, by, with" (compare Old English ge-, German ge-). The prefix in Latin sometimes was used as an intensive.
Before vowels and aspirates, it is reduced to co-; before -g-, it is assimilated to cog- or con-; before -l-, assimilated to col-; before -r-, assimilated to cor-; before -c-, -d-, -j-, -n-, -q-, -s-, -t-, and -v-, it is assimilated to con-, which was so frequent that it often was used as the normal form.
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).