before vowels ped-, word-forming element meaning "boy, child," from Greek pedo-, combining form of pais "boy, child," especially a son, from PIE root *pau- (1) "few, little." The British form paed- is better because it avoids confusion with the ped- that means "foot" (from PIE root *ped-) and the ped-that means "soil, ground, earth." Compare, from the same root, Sanskrit putrah "son;" Avestan puthra- "son, child;" Latin puer "child, boy," Oscan puklu "child."
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).