Douglas Harper's Etymology Dictionary
word-forming element meaning "back to the original place; again, anew, once more," also with a sense of "undoing," c.1200, from Old French and directly from Latin re- "again, back, anew, against," "Latin combining form conceivably from Indo-European *wret-, metathetical variant of *wert- "to turn" [Watkins]. Often merely intensive, and in many of the older borrowings from French and Latin the precise sense of re- is lost in secondary senses or weakened beyond recognition. OED writes that it is "impossible to attempt a complete record of all the forms resulting from its use," and adds that "The number of these is practically infinite ...." The Latin prefix became red- before vowels and h-, as in redact, redeem, redolent, redundant.
The Collaborative International Dictionary
Re- \Re-\ (r[=e]-). [L. re-, older form (retained before vowels) red-: cf. F. re-, r['e]-.] A prefix signifying back, against, again, anew; as, recline, to lean back; recall, to call back; recede; remove; reclaim, to call out against; repugn, to fight against; recognition, a knowing again; rejoin, to join again; reiterate; reassure. Combinations containing the prefix re- are readily formed, and are for the most part of obvious signification.
pre. 1 again, anew 2 backward