Proto-Indo-European root meaning "two."

It forms all or part of: anadiplosis; balance; barouche; between; betwixt; bezel; bi-; binary; bis-; biscuit; combination; combine; deuce; deuterium; Deuteronomy; di- (1) "two, double, twice;" dia-; dichotomy; digraph; dimity; diode; diphthong; diploid; diploma; diplomacy; diplomat; diplomatic; diplodocus; double; doublet; doubloon; doubt; dozen; dual; dubious; duet; duo; duodecimal; duplex; duplicate; duplicity; dyad; epididymis; hendiadys; pinochle; praseodymium; redoubtable; twain; twelfth; twelve; twenty; twi-; twice; twig; twilight; twill; twin; twine; twist; 'twixt; two; twofold; zwieback.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit dvau, Avestan dva, Greek duo, Latin duo, Old Welsh dou, Lithuanian dvi, Old Church Slavonic duva, Old English twa, twegen, German zwei, Gothic twai "two;" first element in Hittite ta-ugash "two years old."

word-forming element, Latin -plus "-fold." Watkins derives it from *-plo-, combining form of PIE root *pel- (2) "to fold" and makes it cognate with Old English -feald, Greek -paltos, -plos. But de Vaan connects it to PIE root *pele- (1) that yielded words meaning "much, many, more" and is the source of poly-.

late 15c., in chess, "to attack the king; to put (the opponent's king) in check;" earlier (late 14c.) in a figurative sense, "to stop, arrest; block, barricade;" from check (n.1) or Old French eschequier, from the noun in French. A player in chess limits his opponent's ability to move when he places his opponent's king in check.

The other senses seem all to have developed from the chess sense, or from the noun: "To arrest, stop," then "to hold in restraint" (1620s); "to hold up or control" (an assertion, a person, etc.) by comparison with some authority or record (1690s); of baggage, etc., "to hand over in return for a check that serves as a means of identifying" (1846); "to note with a mark as having been examined, etc., mark off from a list" (1928).

Hence, to check off (1839); to check up (1883); to check in or out (in a hotel, of a library book, etc., 1909). To check out (something) "to look at, investigate" is from 1959. Related: Checked; checking.

"add or alter sound on film," 1929, shortening of double (v.); so called because it involves making an additional recording of voices and combining it with the soundtrack. The type of re-mixed reggae music was so called from 1974, probably for the same reason. Related: Dubbed; dubbing.

mid-15c., redoublen, "double (something) again or repeatedly, multiply" (transitive), from Old French redobler, redoubler, from re- "again" (see re-) + doubler "to double" (see double (v.)). Intransitive meaning "become twice as much" is from late 15c. Related: Redoubled; redoubling.

"apparition of a living person, 1826 (from 1824 as a German word in English), from German Doppelgänger, literally "double-goer," originally with a ghostly sense. See double + gang (n.). Sometimes half-Englished as doubleganger.

"spy who works for two mutually hostile countries," but usually is loyal to only one of them, by 1920, from double (adj.) + agent (n.).

"date involving two couples," by 1922, American English, from double (adj.) + date (n.3). As a verb by 1938. Related: Double-dating.

"gibberish, incomprehensible language," by 1847 (High Dutch for "incomprehensible language" is recorded by 1789); from double (adj.) + Dutch.

in reference to morality and ethics, "rule, principle, or judgment viewed as applying more strictly to one set of people or circumstance than another," 1871; see double (adj.) + standard (n.). Not common before mid-20c. (especially of codes of sexual behavior for women more strict than those for men); earlier it referred to monetary policy and bimetallism (1823).