Online Etymology Dictionary

D: fourth letter of the Roman alphabet, from Greek delta, from Phoenician and Hebrew daleth, pausal form of deleth "door," so called from its shape. The sign for "500" in Roman numerals. 3-D for "three-dimensional" is attested from 1952.
D-day (n.): 1918, "date set for the beginning of a military operation," with D as an abbreviation of day; compare H-hour, also from the same military order of Sept. 7, 1918:
The First Army will attack at H-Hour on D-Day with the object of forcing the evacuation of the St. Mihiel salient. [Field Order No. 8, First Army, A.E.F.]
"They designate the day and hour of the operation when the day and hour have not yet been determined, or where secrecy is essential" [U.S. Army Center of Military History Web site]. Now almost exclusively of June 6, 1944.
D.A.: American English initialism (acronym) for district attorney from 1934; for duck's ass haircut (or, as OED would have it, duck's arse), from 1951. The haircut so called for the shape at the back of the head.
d.c.: abbreviation of direct current, attested from 1898.
D.C.: abbreviation of District of Columbia, apparently not widely used before 1820, but eventually it became necessary to distinguish the place from the many other "Washingtons" in America. The city and the district were named in 1791 (at first known as Territory of Columbia; the territory was organized as a "district" in 1801), but the towns within it (Washington, Georgetown, Alexandria) remained separate municipalities and at one time all took D.C. The district was effectively organized as a unitary municipality in 1871.
D.D.: abbreviation of Latin Divinitatis Doctor "Doctor of Divinity."
D.D.T.: also DDT, 1943, from dichlorodiphenyltrichlorethane; first made in U.S. by Geigy Co.
D.T.: 1858, abbreviation of delirium tremens.
dab (v.): c. 1300, dabben "to strike," of unknown origin, perhaps imitative. Modern sense of "strike with a slight, quick pressure" developed by mid-16c., influenced by French dauber (see daub). Related: Dabbed; dabbing. As a noun from c. 1300, "heavy blow with a weapon." Dab hand is British slang, 1828, from dab "expert" (1690s), said to be school slang, of unknown origin, perhaps from dab in the "strike lightly" sense.
dabble (v.): 1550s, probably a frequentative of dab. Original meaning was "wet by splashing;" modern figurative sense of "do superficially" first recorded 1620s. Related: Dabbled; dabbling. An Ellen Dablewife is in the Lancashire Inquests from 1336.
dace (n.): small, freshwater fish, early 15c., from Old French darz, nominative or plural of dart "dart" (see dart (n.)). So called for its movements. But another theory traces it to a Medieval Latin darsus, said to be of Gaulish origin.
dacha (n.): from Russian dacha, originally "gift," from Slavic *datja, from PIE *do- "to give" (see donation).
Dachau: town in Bavaria, Germany, from Old High German daha "clay" + ouwa "island," describing its situation on high ground by the Amper River. Infamous as the site of a Nazi concentration camp nearby, opened in 1933 as a detention site for political prisoners and surrendered to the U.S. Army April 29, 1945. Not a death camp per se, but as it was one of the places where inmates from other camps were sent as the Reich collapsed at the end of the war, and as it was one of the few large camps overrun by British or American forces, it has come to symbolize Nazi atrocities in many minds in the West. "Arbeit Macht Frei" was spelled out in metal on the gate (as it was on other concentration camps, such as Gross-Rosen, Sachsenhausen, Theresienstadt).
dachshund (n.): 1881, from German Dachshund (15c.), from Dachs (Old High German dahs, 11c.) "badger" (perhaps literally "builder;" see texture) + Hund "dog" (see hound (n.)). Probably so called because the dogs were used in badger hunts, their long, thin bodies bred to burrow into setts. French taisson, Spanish texon, tejon, Italian tasso are Germanic loan words.
Dacron (n.): 1951, proprietary name (reg. U.S. Patent Office) by E.I. du Pont de Nemours & Co.; an invented word, on the model of nylon, etc.
dactyl (n.): metrical foot, late 14c., from Greek dactylos, literally "finger" (also "toe"), which is of unknown origin; the metrical use (a long syllable followed by two short ones) is by analogy with the three joints of a finger.
dactylic (adj.): 1580s, from Latin dactylicus, from Greek daktylikos "pertaining to a dactyl," from daktylos (see dactyl).
dad (n.): recorded from c. 1500, but probably much older, from child's speech, nearly universal and probably prehistoric (compare Welsh tad, Irish daid, Czech, Latin, Greek tata, Lithuanian tete, Sanskrit tatah, all of the same meaning).
dada: 1920, from French dada "hobbyhorse," child's nonsense word, selected 1916 by Romanian poet Tristan Tzara (1896-1963), leader of the movement, for its resemblance to meaningless babble.
Freedom: DADA DADA DADA, the howl of clashing colors, the intertwining of all contradictions, grotesqueries, trivialities: LIFE. [T. Tzara, "Dada Manifesto," 1918]
Related: Dadaist; Dadaism.
daddy (n.): c. 1500, colloquial diminutive of dad, with -y (3). Daddylonglegs is from 1814; daddy-o is first recorded 1949, from bop talk.
dado (n.): 1660s, of pedestals, from Italian dado "die, cube," from Latin datum (see die (n.)). Of wood panelling in a room, from 1787.
DAE: initialism (acronym) for "A Dictionary of American English on Historical Principles," published in four volumes between 1936 and 1944, edited by Sir William A. Craigie and James R. Hulbert.
daedal (adj.): 1580s, "skillful, cunning," from Latin daedalus, from Greek daidalos "skillful, cunningly wrought." Also an Englished form of the name Daedalus from Greek mythology (1610s).
Daedalus: father of Icarus in Greek mythology, builder of the Cretan labyrinth, from Greek Daidalos, literally "the cunning worker," from daidallein "to work artfully."
daemon (n.): alternative spelling (in specialized senses) of demon (q.v.). Related: Daemonic.
daffodil (n.): 1540s, variant of Middle English affodill "asphodel" (c. 1400), from Medieval Latin affodillus, from Latin asphodelus, from Greek asphodelos, which is of unknown origin. The initial d- is perhaps from merging of the article in Dutch de affodil, the Netherlands being a source for bulbs. First reference to the flower we know by this name (Narcissus pseudo-Narcissus) is from 1590s. Lent-lily for "daffodil" is from 1827.
daffy (adj.): 1884, perhaps from daft (adj.), or from obsolete daffe "a halfwit" (early 14c.; mid-13c. as a surname). Compare late 15c. daffish "dull-witted, spiritless." With -y (2). Related: Daffily; daffiness.
daft (adj.): Old English gedæfte "gentle, becoming," from Proto-Germanic *gadaftjaz (source also of Old English daeftan "to put in order, arrange," gedafen "suitable;" Gothic gadaban "to be fit"), from PIE *dhabh- "to fit together" (see fabric). Sense of "mild, well-mannered" (c. 1200) led to that of "dull, awkward" (c. 1300). Further evolution to "foolish" (mid-15c.), "crazy" (1530s) probably was influenced by analogy with daffe "halfwit" (see daffy); the whole group probably has a common origin.
dag (n.): "thin rain, drizzle, wet fog," late 17c., from a Scandinavian source such as Old Norse dögg, plural daggir "dew," from Proto-Germanic *daowo- (source of Old English deaw; see dew).
dagga (n.): "marijuana," 1660s, from Afrikaans, from Hottentot dachab "cannabis sativa smoked as a narcotic."
dagger (n.): late 14c., apparently from Old French dague "dagger," from Old Provençal dague or Italian daga, which is of uncertain origin; perhaps Celtic, perhaps from Vulgar Latin *daca "Dacian knife," from the Roman province in modern Romania. The ending is possibly the faintly pejorative -ard suffix. Attested earlier (1279) as a surname (Dagard, presumably "one who carried a dagger"). Also compare dogwood. Middle Dutch dagge, Danish daggert, German Degen also are from French.
dago (n.): 1823, from Spanish Diego "James." Originally used of Spanish or Portuguese sailors on English or American ships; by 1900 it had broadened to include non-sailors and shifted to mean chiefly "Italian." James the Greater is the patron saint of Spain, and Diego as generic for "a Spaniard" is attested from 1610s.
Dagon (n.): god of the Philistines, from Hebrew Dagon, from dag "fish."
daguerreotype (n.): 1839, from French daguerreotype, coined from name of inventor, Louis Daguerre (1789-1851) + -type (see type (n.)).
dahlia (n.): 1804, named 1791 by Spanish botanist Antonio José Cavanilles for Anders Dahl (1751-1789), Swedish botanist and pupil of Linnaeus, who discovered it in Mexico in 1788. The likelihood that a true blue variety of the flower never could be cultivated was first proposed by French-Swiss botanist Augustin Pyramus de Candolle, and noted in English by 1835; hence blue dahlia, figurative expression for "something impossible or unattainable" (1866).
daily (adj.): Old English dæglic (see day). This form is known from compounds: twadæglic "happening once in two days," þreodæglic "happening once in three days;" the more usual Old English word was dæghwamlic, also dægehwelc. Cognate with German täglich.
daimon (n.): transliteration of Greek daimon "lesser god, guiding spirit, tutelary deity," 1852; see demon. Employed to avoid the post-classical associations of that word.
daimyo: also daimio, former title of the chief nobles of Japan, 1839, from Japanese, literally "big name," from Chinese dai "great" + mio, myo "name."
daintily (adv.): c. 1300, "sumptuously;" late 14c., "elegantly," from dainty (adj.) + -ly (2).
dainty (n.): c. 1300, "excellence, elegance; a luxury," from Old French deintie (12c.) "price, value," also "delicacy, pleasure," from Latin dignitatem (nominative dignitas) "greatness, rank, worthiness, worth, beauty," from dignus "worthy" (see dignity).
dainty (adj.): c. 1300, deinte, "delightful, pleasing," from dainty (n.). Meaning evolved in Middle English to "choice, excellent" (late 14c.) to "delicately pretty." Related: Daintiness.
daiquiri (n.): type of alcoholic drink, 1920 (first recorded in F. Scott Fitzgerald), from Daiquiri, name of a district or village in eastern Cuba.
dairy (n.): late 13c., "building for making butter and cheese; dairy farm," formed with Anglo-French -erie affixed to Middle English daie (in daie maid "dairymaid"), from Old English dæge "kneader of bread, housekeeper, female servant" (see dey (n.1)). The purely native word was dey-house.
dais (n.): mid-13c., from Anglo-French deis, Old French dais "table, platform," from Latin discus "disk-shaped object," also, by medieval times, "table," from Greek diskos "quoit, disk, dish" (see disk (n.)). Died out in English c. 1600, preserved in Scotland, revived 19c. by antiquarians.
daisy (n.): Old English dægesege, from dæges eage "day's eye," because the petals open at dawn and close at dusk. (See day (n.) + eye (n.)). In Medieval Latin it was solis oculus "sun's eye." As a female proper name said to have been originally a pet form of Margaret (q.v.).

Daisy-cutter first attested 1791, originally of horses that trot with low steps; later of cricket (1889) and baseball hits that skim along the ground. Daisy-chain in the "group sex" sense is attested from 1941. Pushing up daisies "dead" is attested from 1918, but variants with the same meaning go back to 1842.
Dakota: 1809, name of a group of native peoples from the Plains states speaking a Siouan language, from Dakota dakhota "friendly" (the name often is translated as "allies"). Recorded by Lewis and Clark (1804) as Dar co tar; in western dialects of the Teton subgroup, Lakota, Lakhota; in Assiniboine dialect, Nakota, Nakhota.
dal (n.): 1690s, from Hindi dal "split pulse," from Sanskrit dala, from dal "to split."
Dalai Lama: literally "the Ocean Lama," from Mongolian dalai "ocean" + lama.
dale (n.): Old English dæl "dale, valley, gorge," from Proto-Germanic *dalan "valley" (source also of Old Saxon, Dutch, Gothic dal, Old Norse dalr, Old High German tal, German Tal "valley"), from PIE *dhel- "a hollow" (source also of Old Church Slavonic dolu "pit," Russian dol "valley"). Preserved by Norse influence in north of England.
daliance (n.): mid-14c., "confab, chat," from dally + -ance. Probably formed in Anglo-French, but not attested there. Meaning "amorous play, flirtation" is from late 14c.; that of "idle or frivolous activity" is from 1540s.