Longman Dictionary of Contemporary English
The Collaborative International Dictionary
Retro- \Retro-\ [L. retro, adv., backward, back. Cf. Re-.] A prefix or combining form signifying backward, back; as, retroact, to act backward; retrospect, a looking back.
Douglas Harper's Etymology Dictionary
word-forming element meaning "backwards; behind," from Latin retro (prep.) "backward, back, behind," also of time, "formerly, in past times," probably originally the ablative form of *reteros, based on re- "back" (see re-).\n\nL. retro stands to re- as intro, "in, within"; to in, "in," and as citro, "hither," stands to cis, "on this side." [Klein]\nCommon in combinations in post-classical Latin (the classical equivalent was post-). Active in English as a word-forming element from mid-20c.
Wiktionary
pre. 1 back or backward 2 behind 3 in the opposite direction 4 (context informal English) in an old-fashioned or old school way