late 13c., allien, "join in marriage" (transitive), from Old French alier "combine, unite," from a differentiated stem of aliier (from Latin alligare "bind to, tie to," from ad "to" (see ad-) + ligare "to bind, bind one thing to another, tie" (from PIE root *leig- "to tie, bind"). The meaning "form an alliance, join, associate" is late 14c. Related: allied; allying.

"rectangular groove or channel cut out of the edge of a board or piece of stone so that it will join by overlapping with the next piece, similarly cut," late 14c., rabet, from Old French rabat "a recess in a wall, a lower section," literally "a beating down or back," a back-formation from rabattre "to beat down, beat back" (see rebate (v.); the noun is a doublet of this word).

The verb, meaning "to groove or fit (boards, etc.) by cutting rabbets" is attested from mid-15c. (implied in rabatted, rabetynge); Middle English also had rabet "joiner's plane, rabbet-plane" (mid-15c., from Old French rabot).

"complain, speak vehemently and bitterly," late 15c., railen, from Old French raillier "to tease or joke" (15c.), which is perhaps from Old Provençal ralhar "scoff, to chat, to joke," from Vulgar Latin *ragulare "to bray" (source also of Italian ragghiare "to bray"), from Late Latin ragere "to roar," probably of imitative origin. See rally (v.2). Related: Railed; railing.

mountaineering technique for descending steep faces, 1931, from French rappel, literally "recall" (Old French rapel), from rapeler "to recall, summon" (see repeal (v.), which is a doublet). The same word had been borrowed into English earlier to mean "a drum roll to summon soldiers" (1848). For spelling, see rally (v.1).

"establishment of cordial relations," 1809, from French rapprochement "reunion, reconciliation," literally "a bringing near," from rapprocher "bring near," from re- "back, again" (see re-) + aprocher (see approach (v.)). Generally in italics until 1880s.

late 14c., rebaten, "to reduce, diminish;" early 15c., "to deduct, subtract," from Old French rebatre, rabatre, rabattre "beat down, drive back," also "deduct," from re-, "back," or perhaps "repeatedly" (see re-) + abattre "beat down" (see abate).

Original senses now are obsolete. The meaning "to pay back (a sum) as a rebate" is from 1957 and might be from rebate (v.). Related: Rebated; rebating.

late 14c., repēlen, "revoke, rescind, annul; withdraw (a privilege, etc.); repudiate (one's behavior)," from Anglo-French repeler (mid-14c.), Old French rapeler "call back, call in, call after, revoke" (Modern French rappeler), from re- "back" (see re-) + apeler "to call" (later appeler; see appeal (v.)). Related: Repealed; repealing; repealable.

"to form an alliance again, to connect or unite again," c. 1600, from re- + ally (v.). A doublet of rally (v.1), with hyphenated spelling and full pronunciation of the prefix to distinguish it from really. Related: Re-allied; re-alliance.

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 tie, bind." 

It forms all or part of: alloy; ally; colligate; deligate; furl; league (n.1) "alliance;" legato; liable; liaison; lien; lictor; ligand; ligament; ligate; ligation; ligature; oblige; rally (v.1) "bring together;" religion; rely.

It is the hypothetical source of/evidence for its existence is provided by: Latin ligare "to bind;" Albanian lidh "I bind," and possibly Middle Low German lik "band," Middle High German geleich "joint, limb."