c. 1300, afere, "what one has to do, ordinary business," from Anglo-French afere, Old French afaire "business, event; rank, estate" (12c., Modern French affaire), from the infinitive phrase à faire "to do," from Latin ad "to" (see ad-) + facere "to do, make" (from PIE root *dhe- "to set, put").
Originally a Northern word according to OED, it was brought into general use and given a French spelling by Caxton (late 15c.). The general sense of "vague proceedings" (in romance, war, etc.) is attested by 1702. The meaning "an affair of the heart; a passionate episode" is from French affaire de coeur (itself attested as a French phrase in English from 1809); to have an affair with someone in this sense is by 1726, earlier have an affair of love:
'Tis manifeſtly contrary to the Law of Nature, that one Woman ſhould cohabit or have an Affair of Love with more than one Man at the ſame time. ["Pufendorf's Law of Nature and Nations," transl. J. Spavan, London, 1716]
late 14c., "conflict, fighting; difficulty, trouble," a contraction of at do, literally "to do," a dialectal formation in the Norse-influenced areas of northern England: some Scandinavian languages used at with the infinitive of a verb where Modern English uses to.
From use of the infinitive in much ado ("much to do") and similar phrases, ado came to be regarded as a noun. Compare the sense evolution in to-do and affair (from French infinitive phrase à faire "to do"). The weakened meaning "fuss" is from early 15c. Also used in Middle English for "dealings, traffic," and "sexual intercourse" (both c. 1400).
*dhē-, Proto-Indo-European root meaning "to set, put."
It forms all or part of: abdomen; abscond; affair; affect (v.1) "make a mental impression on;" affect (v.2) "make a pretense of;" affection; amplify; anathema; antithesis; apothecary; artifact; artifice; beatific; benefice; beneficence; beneficial; benefit; bibliothec; bodega; boutique; certify; chafe; chauffeur; comfit; condiment; confection; confetti; counterfeit; deed; deem; deface; defeasance; defeat; defect; deficient; difficulty; dignify; discomfit; do (v.); doom; -dom; duma; edifice; edify; efface; effect; efficacious; efficient; epithet; facade; face; facet; facial; -facient; facile; facilitate; facsimile; fact; faction (n.1) "political party;" -faction; factitious; factitive; factor; factory; factotum; faculty; fashion; feasible; feat; feature; feckless; fetish; -fic; fordo; forfeit; -fy; gratify; hacienda; hypothecate; hypothesis; incondite; indeed; infect; justify; malefactor; malfeasance; manufacture; metathesis; misfeasance; modify; mollify; multifarious; notify; nullify; office; officinal; omnifarious; orifice; parenthesis; perfect; petrify; pluperfect; pontifex; prefect; prima facie; proficient; profit; prosthesis; prothesis; purdah; putrefy; qualify; rarefy; recondite; rectify; refectory; sacrifice; salmagundi; samadhi; satisfy; sconce; suffice; sufficient; surface; surfeit; synthesis; tay; ticking (n.); theco-; thematic; theme; thesis; verify.
It is the hypothetical source of/evidence for its existence is provided by: Sanskrit dadhati "puts, places;" Avestan dadaiti "he puts;" Old Persian ada "he made;" Hittite dai- "to place;" Greek tithenai "to put, set, place;" Latin facere "to make, do; perform; bring about;" Lithuanian dėti "to put;" Polish dziać się "to be happening;" Russian delat' "to do;" Old High German tuon, German tun, Old English don "to do."
"one's skill, talent, or calling," 1792, from French métier "trade, profession," from Old French mestier "task, affair, service, function, duty," from Gallo-Roman *misterium, from Latin ministerium "office, service," from minister "servant" (see minister (n.)).
also schmeer, 1961, "bribery," from Yiddish shmir "spread," from shmirn "to grease, smear," from Middle High German smiren, from Old High German smirwen "to smear" (see smear (v.); compare slang grease (someone's) palm "to bribe"). Phrase the whole schmear "the entire affair" is attested by 1969, originally show business jargon,
"in the matter of, in the (legal) case of," c. 1600, probably from Duns Scotus; Latin, from re, ablative of res "property, goods; matter, thing, affair," from Proto-Italic *re-, from PIE *reh-i- "wealth, goods" (source also of Sanskrit rayi- "property, goods," Avestan raii-i- "wealth").
1650s, "of the nature of a literary romance, partaking of the heroic or marvelous," from French romantique "pertaining to romance," from romant "a romance," an oblique case or variant of Old French romanz "verse narrative" (see romance (n.)).
Of places, "characterized by poetic or inspiring scenery," by 1705. As a literary style, opposed to classical (q.v.) since before 1812; it was used of schools of poetry in Germany (late 18c.) and later France. In music, "characterized by expression of feeling more than formal methods of composition," from 1885. Meaning "characteristic of an ideal love affair" (such as usually formed the subject of literary romances) is from 1660s. Meaning "having a love affair as a theme" is from 1960. Related: Romantical (1670s); romantically; romanticality. Compare romanticism.