Online Etymology Dictionary

duo (n.): 1580s, "song for two voices," via either Italian or French, from Latin duo "two" (see two). Meaning "two people" (especially as an entertainment team) attested by 1887.
trio (n.): 1724, "composition for three voices," from French trio (c. 1600), from Italian trio, from tri- "three" (see three); patterned on duo. Meaning "group of three persons" is from 1789.
dual (adj.): c. 1600, from Latin dualis, from duo "two" (see two). Related: Dually.
duel (n.): 1590s (from late 13c. in Latin form), from Medieval Latin duellum "combat between two persons," by association with Latin duo "two," but originally from Latin duellum "war," an Old Latin form of bellum (see bellicose). Retained in poetic and archaic language and apparently given a special meaning in Medieval or Late Latin of "one-on-one combat" on fancied connection with duo "two."
di- (1): word-forming element meaning "two, double, twice," from Greek di-, from dis "twice," related to duo (see two).
duet (n.): 1740, from French duet, from Italian duetto "short musical composition for two voices," diminutive of duo "two" (see two). As a verb, from 1822. The Italian form of the noun was used in English from c. 1724.
dia-: before vowels, di-, word-forming element meaning "through, thoroughly, entirely," from Greek dia-, from dia "through, throughout," probably from the root of duo "two" (see two) with a base sense of "twice."
duplicate (adj.): mid-15c., "having two parts, double," from Latin duplicatus, past participle of duplicare "to double," from duo "two" (see two) + plicare "to fold" see ply (v.1)). Meaning "exactly corresponding, that is an exact copy of" is from 1812.
duplex (adj.): 1817, "composed of two parts," from Latin duplex, from duo "two" (see two) + -plex, from Greek plax (genitive plakos) "flat surface." The noun sense of "house for two families; two-story apartment" is American English, 1922.
dyad (n.): 1670s, from Latin dyad-, stem of dyas, from Greek dyas "the number two, a group of two," from duo "two" (see two). Specific sense in chemistry ("a bivalent element") is by 1865; also used in biology, poetics, mathematics. Related: Dyadic.
dodecahedron (n.): 1560s, from Greek dodeka "twelve" (short for duodeka, from duo "two" + deka "ten;" compare dozen) + hedra "seat, base, chair, face of a geometric solid," from PIE root *sed- (1) "to sit" (see sedentary).
deuterium (n.): 1933, coined by U.S. chemist Harold C. Urey, with Modern Latin ending + Greek deuterion, neuter of deuterios "having second place," from deuteros "next, second," according to some sources from duo (see two), but according to Watkins the ground sense is "missing" and the Greek word is from PIE from *deu-tero-, suffixed form of root *deu- "to lack, be wanting." So called because it is twice the mass of hydrogen.
dozen (n.): c. 1300, from Old French dozaine "a dozen," from doze (12c.) "twelve," from Latin duodecim "twelve," from duo "two" + decem "ten" (see ten).

The Old French fem. suffix -aine is characteristically added to cardinals to form collectives in a precise sense ("exactly 12," not "about 12"). The dozens "invective contest" (1928) originated in slave culture, the custom probably African, the word probably from bulldoze (q.v.) in its original sense of "a whipping, a thrashing."
dubious (adj.): 1540s, from Latin dubiosus "doubtful," from dubium "doubt," neuter of dubius "vacillating, moving two ways, fluctuating;" figuratively "wavering in opinion, doubting, doubtful," from duo "two" (see two), with a sense of "of two minds, undecided between two things." Old English also used tweo "two" to mean "doubt." Compare doubt (v.). Related: Dubiously; dubiousness.
double (adj.): early 13c., from Old French doble (10c.) "double, two-fold; two-faced, deceitful," from Latin duplus "twofold, twich as much" from duo "two" (see two) + -plus "more" (see -plus). Double standard attested by 1951. Military double time (1833) originally was 130 steps per minute.
two (adj.): Old English twa "two," fem. and neuter form of twegen "two" (see twain), from Proto-Germanic *twa (source also of Old Saxon and Old Frisian twene, twa, Old Norse tveir, tvau, Dutch twee, Old High German zwene, zwo, German zwei, Gothic twai), from PIE *duwo, variant of dwo "two" (source also of Sanskrit dvau, Avestan dva, Greek duo, Latin duo, Old Welsh dou, Lithuanian dvi, Old Church Slavonic duva "two," first element in Hittite ta-ugash "two years old").

Two-fisted is from 1774. Two cheers for _____, expressing qualified enthusiasm first recorded 1951 in E.M. Forster's title "Two Cheers for Democracy." Two-dimensional is recorded from 1883; figurative sense of "lacking substance or depth" is attested from 1934.
deuce (n.): late 15c., "the 2 in dice or cards," also "a roll of 2 in dice" (1510s), from Middle French deus (Modern French deux), from Latin duos (nominative duo) "two" (see two).

Became a mild oath by 1710, about 50 years after it was first attested in the sense of "bad luck, the devil, etc.," perhaps because two was the lowest score, and probably by similarity to Latin deus and related words meaning "god." Low German had der daus! in same sense 16c., which perhaps influenced the English form. Deuce coupe is 1940s hot-rodder slang for "souped up two-door car," especially a 1932 Ford. Related: Deuced; deucedly.
dis-: (assimilated as dif- before -f-, to di- before most voiced consonants), word-forming element meaning 1. "lack of, not" (as in dishonest); 2. "do the opposite of" (as in disallow); 3. "apart, away" (as in discard), from Old French des- or directly from Latin dis- "apart, in a different direction, between," figuratively "not, un-," also "exceedingly, utterly," from PIE *dis- "apart, asunder" (source also of Old English te-, Old Saxon ti-, Old High German ze-, German zer-).

The PIE root is a secondary form of *dwis- and thus is related to Latin bis "twice" (originally *dvis) and to duo, on notion of "two ways, in twain."

In classical Latin, dis- paralelled de- and had much the same meaning, but in Late Latin dis- came to be the favored form and this passed into Old French as des-, the form used for new compound words formed in Old French, where it increasingly had a privative sense ("not").

In English, many of these words eventually were altered back to dis-, while in French many have been altered back to de-. The usual confusion prevails.
duodecimal (adj.): 1714, from Latin in duodecimo (see duodecimo).
duodenum (n.): late 14c., from Medieval Latin duodenum digitorium "space of twelve digits," from Latin duodeni "twelve each." Coined by Gerard of Cremona (d.1187), who translated "Canon Avicennae," a loan-translation of Greek dodekadaktylon, literally "twelve fingers long," the intestine part so called by Greek physician Herophilus (c. 353-280 B.C.E.) for its length, about equal to the breadth of twelve fingers.
duodecimo: 1650s, from Latin in duodecimo (folded) "in a twelfth" of a sheet, from ablative of duodecimus "twelfth," from duodecim (see dozen). Often abbreviated 12mo.; a book in which each page is the twelfth part of the printer's sheet.
duodenal (adj.): 1817; see duodenum + -al (1).