Online Etymology Dictionary

Q: 16th letter of the classical Roman alphabet, from the Phoenician equivalent of Hebrew koph, qoph, which was used for the more guttural of the two "k" sounds in Semitic.

The letter existed in Greek, but was little used and not alphabetized; the stereotypical connection with -u- began in Latin. Anglo-Saxon scribes adopted the habit at first, but later used spellings with cw- or cu-. The qu- pattern returned to English with the Norman Conquest and had displaced cw- by c. 1300. In some spelling variants of late Middle English, quh- also took work from wh-, especially in Scottish and northern dialects, for example Gavin Douglas, Provost of St. Giles, in his vernacular "Aeneid" of 1513:
Lyk as the rois in June with hir sueit smell
The marygulde or dasy doith excell.
Quhy suld I than, with dull forhede and vane,
With ruide engine and barrand emptive brane,
With bad harsk speche and lewit barbour tong,
Presume to write quhar thi sueit bell is rong,
Or contirfait sa precious wourdis deir?
Scholars use -q- alone to transliterate Semitic koph (as in Quran, Qatar, Iraq ). In Christian theology, Q has been used since 1901 to signify the hypothetical source of passages shared by Matthew and Luke, but not in Mark; in this sense probably it is an abbreviation of German Quelle "source."
Q and A: also Q & A, 1954, abbreviation of question and answer (itself attested by 1817).
Q.E.D.: 1760, abbreviation of Latin quod erat demonstrandum "which was to be demonstrated."
q.t. (n.): slang for "quiet," in phrase on the q.t., attested from 1874. Phrase on the quiet appears from 1847.
Qatar: probably from Arabic katran "tar, resin," in reference to petroleum. Related: Qatari.
qi (n.): "physical life force," 1850, from Chinese qi "air, breath."
qu-: see Q.
qua: "as, in the capacity of," from Latin qua "where? on which side? at which place? which way? in what direction?" figuratively "how? in what manner? by what method?; to what extent? in what degree?" correlative pronominal adverb of place, from PIE *kwo-, stem of relative and interrogative pronouns (source also of Old English hwa "who;" see who).
Quaalude (n.): 1965, proprietary name (trademark by Wm. H. Rohrer Inc., Ft. Washington, Pennsylvania, U.S.A.) of methaqualone.
quack (v.): "to make a duck sound," 1610s, earlier quake (1520s), variant of quelke (early 14c.), of echoic origin (compare Middle Dutch quacken, Old Church Slavonic kvakati, Latin coaxare "to croak," Greek koax "the croaking of frogs," Hittite akuwakuwash "frog"). Middle English on the quakke (14c.) meant "hoarse, croaking." Related: Quacked; quacking.
quack (n.1): "medical charlatan," 1630s, short for quacksalver (1570s), from obsolete Dutch quacksalver (modern kwakzalver), literally "hawker of salve," from Middle Dutch quacken "to brag, boast," literally "to croak" (see quack (v.)) + salf "salve," salven "to rub with ointment" (see salve (v.)). As an adjective from 1650s. The oldest attested form of the word in this sense in English is as a verb, "to play the quack" (1620s). The Dutch word also is the source of German Quacksalber, Danish kvaksalver, Swedish kvacksalvare.
quack (n.2): duck sound, 1839, from quack (v.).
quacker (n.): "a duck," 1846, agent noun from quack (v.).
quackery (n.): 1690s, from quack (n.) + -ery.
quacksalver (n.): 1570s; see quack (n.1).
quad: 1820 as a shortening of quadrangle (n.) in the building sense (in this case "quadrangle of a college," Oxford student slang); 1880 as short for quadrat (n.); 1896 as quadruplet (n.), originally "bicycle for four riders;" later "one of four young at a single birth" (1951, of armadillos); 1970 as quadraphonic (adj.). Related: Quads.
Quadragesima (n.): c. 1600, from Medieval Latin quadragesima (dies) "the fortieth (day)," altered diminutive of Latin quadrigesimus "fortieth," from quadriginta "forty," related to quattuor "four" (see four). Related: Quadragesimal.
quadrangle (n.): late 14c., from Old French quadrangle (13c.) and directly from Late Latin quadrangulum "four-sided figure," noun use of neuter of Latin adjective quadrangulus "having four quarters," from Latin quattuor "four" (see four) + angulus "angle" (see angle (n.)). Meaning "four-sided court between buildings" is from 1590s.
quadrangular (adj.): early 15c., from Medieval Latin quadrangularis "having four corners," from Late Latin quadrangulus (see quadrangle).
quadrant (n.): late 14c., "a quarter of a day, six hours," from Middle French quadrant, from Latin quadrantem (nominative quadrans) "fourth part," also the name of a coin worth a quarter of an as, noun use of present participle of quadrare "to make square; put in order, arrange, complete; run parallel, be exact," figuratively "to fit, suit, be proper," related to quadrus "a square," quattuor "four" (see four). The surveying instrument is first so called c. 1400, because it forms a quarter circle. Related: Quadrantal.
quadraphonic (adj.): 1969, irregular formation from quadri- "four" + phonic, from Greek phone "sound, voice," from PIE root *bha- (2) "to speak, tell, say" (see fame (n.)). The goal was to reproduce front-to-back sound distribution in addition to side-to-side stereo. The later term for the same idea, surround sound, is preferable to this.
quadrat (n.): "a blank, low-cast type used by typographers to fill in larger spaces in printed lines," 1680s, from French quadrat "a quadrat," literally "a square," from Latin quadratrus, past participle of quadrare "to square, make square" (see quadrant). Earlier in English it meant a type of surveying instrument (c. 1400).
quadratic (adj.): 1650s, "square," with -ic + obsolete quadrate "a square; a group of four things" (late 14c.), from Latin quadratum, noun use of neuter adjective quadratus "square, squared," past participle of quadrare "to square, set in order, complete" (see quadrant). Quadratic equations (1660s) so called because they involve the square of x.
quadratus (n.): "square-shaped muscle," 1727, from Latin quadratus "square, squared" (see quadratic).
quadrennial (adj.): 1650s, "lasting four years;" as "happening once every four years," 1701; from quadri- + ending from biennial, etc. Correct formation would be quadriennial (compare Latin quadriennium "period of four years"). As a noun from 1640s. Related: Quadrennially.
quadri-: before vowels quad- (before -p- often quadru-, from an older form in Latin), word-forming element meaning "four, four times, having four, consisting of four," from Latin quadri-, related to quattor "four" (see four).
quadricentennial (n.): also quadri-centennial, 1859, from quadri- + centennial.
quadricep (n.): large extensor muscle of the leg, 1840, from quadri- on model of bicep. Related: Quadriceps. So called because divided into four parts.
quadriceps (n.): 1840, see quadricep.
quadrifid (adj.): "divided in four parts," 1660s, from quadri- + -fid.
quadrilateral (n.): "four-sided," 1640s, with -al (1) + Latin quadrilaterus, from quadri- "four" (see quadri-) + latus (genitive lateris) "side" (see oblate (n.)). As an adjective from 1650s. Related: Quadrilaterally.
quadriliteral (adj.): "consisting of four letters," 1771, from quadri- + literal.
quadrille (n.): 1773, "lively square dance for four couples," from French quadrille (17c.), originally one of four groups of horsemen in a tournament (a sense attested in English from 1738), from Spanish cuadrilla, diminutive of cuadro "four-sided battle square," from Latin quadrum "a square," related to quattuor "four" (see four). The craze for the dance hit England in 1816, and it underwent a vigorous revival late 19c. among the middle classes.

Earlier the name of a popular card game for four hands, and in this sense from French quadrille (1725), from Spanish cuartillo, from cuarto "fourth," from Latin quartus. OED notes it as fashionable from 1726 ("and was in turn superseded by whist"), the year of Swift's (or Congreve's) satirical ballad on the craze:
The commoner, and knight, the peer,
Men of all ranks and fame,
Leave to their wives the only care,
To propagate their name;
And well that duty they fulfil
When the good husband's at Quadrille &c.
quadrillion (n.): 1670s, from French quadrillion (16c.) from quadri- "four" (see quadri-) + (m)illion. Compare billion. In Great Britain, the fourth power of a million (1 followed by 24 zeroes); in the U.S., the fifth power of a thousand (1 followed by 15 zeroes).
quadripartite (adj.): early 15c., from Latin quadripartitus, from quadri- "four" (see quadri-) + partitus, past participle of partiri "to divide" (see part (v.)).
quadriplegia (n.): 1895, a medical hybrid coined from Latin-based quadri- "four" + -plegia, as in paraplegia, ultimately from Greek plege "stroke," from root of plessein "to strike" (see plague (n.)). A correct, all-Greek formation would be *tetraplegia.
quadriplegic (adj.): also quadraplegic, 1897, from quadriplegia + -ic. A correct, all-Greek formation would be *tessaraplegic. The noun is first attested 1912, from the adjective.
quadrivium (n.): "arithmetic, music, geometry, astronomy," 1804 (see liberal arts), from Latin quadrivium, which meant "place where four roads meet, crossroads," from quadri- "four" (see quadri-) + via "way, road, channel, course" (see via). The adjective quadrivial is attested from late 15c. in English with the sense "having four roads."
quadroon (n.): 1707, "offspring of a white and a mulatto," from Spanish cuarteron (used chiefly of the offspring of a European and a mestizo), literally "one who has a fourth" (Negro blood), from cuarto "fourth," from Latin quartus (see quart), so called because he or she has one quarter African blood. Altered by influence of words in quadr-. There also was some use in 19c. of quintroon (from Spanish quinteron) "one who is fifth in descent from a Negro; one who has one-sixteenth Negro blood."
quadru-: word-forming element meaning "four, having four, consisting of four," variant of quadri-, especially before -p-, from an older form of the element, which perhaps was influenced later by tri-.
quadruped (n.): 1640s, from French quadrupède (16c.), from Latin quadrupes (genitive quadrupedis) "four-footed, on all fours," also, as a noun, "a four-footed animal," from quadri- "four" (see quadri-) + pes "foot" (see foot (n.)). The adjective is attested from 1741. Related: Quadrupedal (1610s).
quadruple (v.): late 14c., from Middle French quadrupler, from Late Latin quadruplare "make fourfold, multiply by four," from Latin quadruplus (adj.) "quadruple, fourfold" (see quadruple (adj.)).
quadruple (adj.): 1550s, from Middle French quadruple (13c.), from Latin quadruplus "fourfold," from quadri- "four" (see quadru-) + -plus "-fold" (see -plus).
quadruplet (n.): "one of four children at a single birth," 1787; from quadruple (adj.) with ending from triplet. Related: Quadruplets. Musical sense is from 1873.
quadruplex (adj.): 1875, in reference to telegraph systems in which four messages can be wired simultaneously, from quadru- + plex. In classical Latin, quadruplex meant "fourfold, quadruple," as a noun, "a fourfold amount."
quadruplicate (adj.): 1650s, from Latin quadruplicatus, past participle of quadruplicare "make fourfold," from quadri- "four" (see quadri-) + plicare "to fold" (see ply (v.1)).
quadruplicate (v.): 1660s, from Latin quadruplicatus, past participle of quadruplicare "make fourfold," from quadri- "four" (see quadri-) + plicare "to fold" (see ply (v.1)). Related: Quadruplicated; quadruplicating.
quadruplication (n.): 1570s, from Latin quadruplicationem (nominative quadruplicatio), noun of action from past participle stem of quadruplicare (see quadruplicate (v.)).
quaere: Latin imperative of quaerere "to ask, inquire" (see query (v.)). Hence "one may ask" (1530s) as an introduction to a question.
quaff (v.): 1510s (implied in quaffer), perhaps imitative, or perhaps from Low German quassen "to overindulge (in food and drink)," with -ss- misread as -ff-. Related: Quaffed; quaffing. The noun is attested by 1570s, from the verb.