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]

"not to be escaped by struggling," 1620s, from French inéluctable (15c.) or directly from Latin ineluctabilis "unavoidable, inevitable," from in- "not, opposite of" (see in- (1)) + eluctabilis "that may be escaped from," from eluctari "to struggle out of," from ex "out, out of" (see ex-) + luctari "to struggle" (see reluctance).

"tress of hair," Old English locc "lock of hair, curl" (plural loccas), from Proto-Germanic *lukkoz (source also of Old Norse lokkr, Old Saxon, Old Frisian, Dutch lok, Old High German loc, German Locke "lock of hair"), a word of uncertain origin. According to OED, perhaps from a PIE *lugnos- and related to Greek lygos "pliant twig, withe," Lithuanian lugnas "flexible" (see reluctance).

c. 1300, "sexual intercourse;" mid-14c., "lasciviousness, sinful self-indulgence;" late 14c., "sensual pleasure," from Old French luxurie "debauchery, dissoluteness, lust" (12c., Modern French luxure), from Latin luxuria "excess, extravagant living, profusion; delicacy" (source also of Spanish lujuria, Italian lussuria), from luxus "excess, extravagance; magnificence," probably a figurative use of luxus (adj.) "dislocated," which is related to luctari "wrestle, strain" (see reluctance).

The English word lost its pejorative taint 17c. Meaning "habit of indulgence in what is choice or costly" is from 1630s; that of "sumptuous surroundings" is from 1704; that of "something choice or comfortable beyond life's necessities" is from 1780. Used as an adjective from 1916.

In Lat. and in the Rom. langs. the word connotes vicious indulgence, the neutral sense of the Eng. 'luxury' being expressed by L. luxus, F. luxe, Sp. lujo, It. lusso. [OED]

"unwilling, struggling against duty or a command," 1660s, from Latin reluctantem (nominative reluctans), present participle of reluctari "to struggle against, resist, make opposition," from re- "against" (see re-) + luctari "to struggle, wrestle" (see reluctance). Related: Reluctantly. The Latin word also is the source of Spanish reluchante, Italian riluttante.

Reluctant, literally, struggling back from, implies some degree of struggle either with others who are inciting us on, or between our own inclination and some strong motive, as sense of duty, whether it operates as an impelling or as a restraining influence. [Century Dictionary]