Origin and history of re-

word-forming element meaning "backback fromback to the original place;" also "againanewonce more," also conveying the notion of "undoing" or "backward," etc. (see sense evolution below)c. 1200from Old French re- and directly from Latin re- an inseparable prefix meaning "again; back; anewagainst."

Watkins (2000) describes this as a "Latin combining form conceivably from Indo-European *wret-metathetical variant of *wert- "to turn." De Vaan says the "only acceptable etymology" for it is a 2004 explanation which reconstructs a root in PIE *ure "back."

In earliest Latin the prefix became red- before vowels and h-a form preserved in redactredeemredolentredundantredintegrateandin disguiserender (v.). In some English words from French and Italian re- appears as ra- and the following consonant is often doubled (see rally (v.1)).

The many meanings in the notion of "back" give re- its broad sense-range: "a turning back; opposition; restoration to a former state; "transition to an opposite state." From the extended senses in "again," re- becomes "repetition of an action," and in this sense it is extremely common as a formative element in Englishapplicable to any verb. 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 ...."  

Often merely intensiveand in many of the older borrowings from French and Latin the precise sense of re- is forgottenlost in secondary sensesor weakened beyond recognitionso that it has no apparent semantic content (receiverecommendrecoverreducerecreatereferreligionremainrequestrequire). Also compare 19c. revamp.

There seem to have been more such words in Middle English than aftere.g. recomfort (v.) "to comfortconsole; encourage;" recourse (n.) "a processwaycourse." Recover in Middle English also could mean "obtainwin" (happinessa kingdometc.) with no notion of getting something backalso "gain the upper handovercome; arrive at;" also consider the legal sense of recovery as "obtain (property) by judgment or legal proceedings." 

Anddue to sound changes and accent shiftsre- sometimes entirely loses its identity as a prefix (rebelrelicremnantrestiverest (n.2) "remainder," rally (v.1) "bring together"). In a few words it is reduced to r-as in ransom (a doublet of redemption)rampartetc.

It was used from Middle English in forming words from Germanic as well as Latin elements (rebuildrefillresetrewrite)and was used so even in Old French (regretregardrewardetc.).

Prefixed to a word beginning with ere- is separated by a hyphenas re-establishre-estatere-edifyetc. ; or else the second e has a dieresis over it: asreëstablishreëmbarketc. The hyphen is also sometimes used to bring out emphatically the sense of repetition or iteration : assung and re-sung. The dieresis is not used over other vowels than e when re is prefixed : thusreinforcereunitereabolish. [Century Dictionary1895]