Etymology
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twin (adj.)
Old English twinn "consisting of two, twofold, double, two-by-two," from Proto-Germanic *twisnjaz "double" (source also of Old Norse tvinnr "double, twin," Old Danish tvinling, Dutch tweeling, German zwillung), from PIE *dwisno- (source also of Latin bini "two each," Lithuanian dvynu "twins"), from *dwi- "double," from root *dwo- "two." Minneapolis and St. Paul in Minnesota have been the Twin Cities since 1883, but the phrase was used earlier of Rock Island and Davenport (1856).
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twin (v.)
"to combine two things closely, join, couple," late 14c., from twin (adj.). Related: Twinned; twinning. In Middle English, the verb earlier and typically meant "to part, part with, separate from, estrange," etc. (c. 1200), on the notion of making two what was one.
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twin (n.)
c. 1300, from Old English getwinn "double;" getwinnas "twins, two born at one birth," from twinn (see twin (adj.)).
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triplet (n.)

1650s, "three successive lines of poetry," from triple; perhaps patterned on couplet. Extended to a set of three of anything by 1733, and to three children at the same birth by 1787 (another word for this was trin, 1831, on the model of twin). Musical meaning "three notes played in the time of two" is from 1801.

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*dwo- 

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."

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Thomas 
masc. proper name, from Greek Thomas, of Aramaic origin and said to mean "a twin" (John's gospel refers to Thomas as ho legomenos didymos "called the twin;" compare Syriac toma "twin," Arabic tau'am "twin"). Before the Conquest, found only as the name of a priest, but after 1066, one of the most common given names in English. Also see Tom, Tommy. Doubting Thomas is from John xx.25. A Thomist (1530s, from Medieval Latin Thomista, mid-14c.) is a follower of 13c. scholastic theologian St. Thomas Aquinas.
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loneling (n.)
"single child" (as opposed to a twin, etc.), 1570s, from lone (adj.) + -ling.
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fraternal (adj.)
early 15c., from Old French fraternel "brotherly, fraternal," and directly from Medieval Latin fraternalis, from Latin fraternus "friendly, closely allied," literally "brotherly" (see fraternity). The noun meaning "fraternal twin" is recorded by 1911.
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gimbal (n.)
1570s, "joints, connecting links," alteration of gemel "twins" (late 14c.), from Old French jumel "a twin" (12c., Modern French jumeau), from Latin gemellus, diminutive of geminus (adj.) "twin, born together" (see geminate). As a type of contrivance for securing free motion in suspension, by 1780. Related: Gimbals. Gemmels (plural) was Middle English for "twins" (late 14c.), also "Gemini," from Old French gemeles; hence also gemel ring, a double finger-ring that may be taken apart; also gimmal.
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twine (n.)
"strong thread made from twisted strands," Old English twin "double thread," from Proto-Germanic *twiznaz "double thread, twisted thread" (source also of Dutch twijn, Low German twern, German zwirn "twine, thread"), from PIE root *dwo- "two."
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