Etymology
From Proto-Italic *wre- (“again”), which has a parallel in Umbrian re-, but its further etymology is uncertain (OED).
While it carries a general sense of "back" or "backwards", its precise sense is not always clear, and its great productivity in classical Latin has the tendency to obscure its original meaning.
Watkins proposes a metathesis of Proto-Indo-European *wert- (“to turn”), (Can this(+) etymology be sourced?) while de Vaan suggests Proto-Indo-European *ure- (“back”), which may be found in Proto-Slavic *rakъ (“crayfish, lobster”) (tentatively, in an original sense *“looking backwards”) and Albanian rrë- (“back”, preverb), unless the latter is borrowed from Latin.[1]
Prefix
re-
- back, backwards
- un-, de-[2]
- re- + glūtinō (“glue”) → reglūtinō (“unglue, separate”)
- re- + neō (“spin, weave, entwine”) → reneō (“unspin, unravel”)
- re- + gelō (“freeze, congeal”) → regelō (“thaw, unfreeze”)
- again; prefix added to various words to indicate an action being done again, or like the other usages indicated above under English.
Usage notes
The alternative form red- occurs before vowels or h in old formations; it is used with the linking vowel -i- in the word redivīvus. The -d- can be compared to that in sēditiō (compare sē- and sed) and in prōd-, antid-, postid- (alternative forms of prō-, ante-, post-). It may originate from the particle *de[3] or from the use of -d as an archaic ablative singular ending. The use of the form re- before vowels, as in reaedifico, reinvito, is not seen until Late Latin.[3] (See Lewis & Short, A Latin Dictionary, 1897, s.v. "re" and "D").
Before consonants, its usual form is rĕ- with short /e/, but the following consonant is sometimes doubled. In some cases, such as reccidī, the double consonant comes from syncope of an originally reduplicated syllable of the base word: compare the unprefixed form cecidī. In other cases, such as redducō, relligiō, relliquiae, the double consonant may have arisen from preconsonantal use of red-, with assimilation of -d- to the following consonant.
References
De Vaan, Michiel (2008) “re-, red-”, in Etymological Dictionary of Latin and the other Italic Languages (Leiden Indo-European Etymological Dictionary Series; 7), Leiden, Boston: Brill, →ISBN, page 516
R. B. Burnaby (1905) Elegiac Selections from Ovid, page 98
Lindsay, Wallace Martin (1894) The Latin Language, page 591