16th letter of the classical Roman alphabet, occurring in English only before a -u- that is followed by another vowel (with a few exceptions; see below), whether the -u- is sounded or not (pique). The letter is from the Phoenician equivalent of Hebrew koph, qoph, which was used for the deeper and more guttural of the two "k" sounds in Semitic. The letter existed in early Greek (where there was no such distinction), and called koppa, but it was little used and not alphabetized; it mainly served as a sign of number (90).

The connection with -u- began in Latin. Anglo-Saxon scribes at first adopted the habit, but later used spellings with cw- or cu-. The qu- pattern returned to English with the Normans and French after the 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 or the equivalent in Turkish or Iranian (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" (from Old High German quella, from the same Proto-Germanic source as Old English cwiella, cwylla"spring; well"). In Middle English accounts, it is an abbreviation of quadrans "farthing" (mid-15c.). In Roman personal names it is an abbreviation of Quintus.

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Q and A 

also Q & A, 1954 (adj.), abbreviation of question and answer (itself attested by 1817 as a noun, by 1839 as an adjective).

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peninsula-state in the Persian Gulf, probably from Arabic katran "tar, resin," in reference to petroleum. The Romans knew it as Catara. Related: Qatari.

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abbreviation of Latin quod est "which is."

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1760, abbreviation of Latin quod erat demonstrandum "which was to be demonstrated."

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qi (n.)

in Chinese philosophy, "physical life force," 1850, said to be from Chinese qi "air, breath."

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q.t. (n.)

slang for "quiet," in phrase on the q.t., attested from 1874. Phrase on the quiet appears from 1847.

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"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 root *kwo-, stem of relative and interrogative pronouns).

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Quaalude (n.)

1965, proprietary name (trademark by Wm. H. Rorer Inc., Ft. Washington, Pennsylvania, U.S.A.) of methaqualone used as a sedative drug. The name is said to be based on quiet interlude.

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