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

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 redact, redeem, redolent, redundant, redintegrate, and, in disguise, render (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 English, applicable 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 intensive, and in many of the older borrowings from French and Latin the precise sense of re- is forgotten, lost in secondary senses, or weakened beyond recognition, so that it has no apparent semantic content (receive, recommend, recover, reduce, recreate, refer, religion, remain, request, require). There seem to have been more such words in Middle English than after, e.g. recomfort (v.) "to comfort, console; encourage;" recourse (n.) "a process, way, course." Recover in Middle English also could mean "obtain, win" (happiness, a kingdom, etc.) with no notion of getting something back, also "gain the upper hand, overcome; arrive at;" also consider the legal sense of recovery as "obtain (property) by judgment or legal proceedings." 

And, due to sound changes and accent shifts, re- sometimes entirely loses its identity as a prefix (rebel, relic, remnant, restive, rest (n.2) "remainder," rally (v.1) "bring together"). In a few words it is reduced to r-, as in ransom (a doublet of redemption), rampart, etc.

It was used from Middle English in forming words from Germanic as well as Latin elements (rebuild, refill, reset, rewrite), and was used so even in Old French (regret, regard, reward, etc.).

Prefixed to a word beginning with e, re- is separated by a hyphen, as re-establish, re-estate, re-edify, etc. ; or else the second e has a dieresis over it: as, reëstablish, reëmbark, etc. The hyphen is also sometimes used to bring out emphatically the sense of repetition or iteration : as, sung and re-sung. The dieresis is not used over other vowels than e when re is prefixed : thus, reinforce, reunite, reabolish. [Century Dictionary, 1895]

Proto-Indo-European root meaning "to cut, strike, stamp."

It forms all or part of: account; amputate; amputation; anapest; berate; compute; count (v.); depute; deputy; dispute; impute; pave; pavement; pit (n.1) "hole, cavity;" putative; rate (v.1) "to scold;" reputation; repute.

It is the hypothetical source of/evidence for its existence is provided by: Latin pavire "to beat, ram, tread down," putare "to prune;" Greek paiein "to strike;" Lithuanian pjauti "to cut," pjūklas "saw."

also demimonde, "women of equivocal reputation and standing in society," 1855, from French demi-monde "so-so society," literally "half-world," from demi- "half" + monde, from Latin mundus "world" (see mundane).

Popularized by its use as title of a comedy by Alexandre Dumas fils (1824-1895). Dumas' Demi-Monde "is the link between good and bad society ... the world of compromised women, a social limbo, the inmates of which ... are perpetually struggling to emerge into the paradise of honest and respectable ladies" ["Fraser's Magazine," 1855]. Thus not properly used of courtesans, etc.

Compare 18th-century English demi-rep (1749, the second element short for reputation), defined as "a woman that intrigues with every man she likes, under the name and appearance of virtue ... in short, whom every body knows to be what no body calls her" [Fielding].

1705 as abbreviation of reputation (n.); upon rep "I swear it" was a common 18c. slang asseveration. As a shortening of repetition (n.) it is recorded from 1864, originally school slang; as a shortening of representative (n.), especially (but not originally) "sales representative," it is attested from 1896. As an abbreviation of repertory (company) it is recorded from 1925.