Online Etymology Dictionary

devour (v.): early 14c., from Old French devorer (12c.) "devour, swallow up, engulf," from Latin devorare "swallow down, accept eagerly," from de- "down" (see de-) + vorare "to swallow" (see voracity). Related: Devoured; devouring.
devout (adj.): early 13c., from Old French devot "pious, devoted, assiduous," from Latin devotus "given up by vow, devoted," past participle of devovere "dedicate by vow" (see devotion).
dew (n.): Old English deaw, from Proto-Germanic *dawwaz (source also of Old Saxon dau, Old Frisian daw, Middle Dutch dau, Old High German tau, German Tau, Old Norse dögg "dew"), from PIE root *dheu- (2) "to flow" (source also of Sanskrit dhavate "flows, runs").
dew claw (n.): also dew-claw, 1570s, from claw, but the signification of the first element is obscure (see dewlap).
dewdrop (n.): early 14c. (as two words); see dew + drop (n.).
Dewey Decimal system: proposed 1876 by Melvil Dewey (1851-1931) while acting librarian of Amherst College. He also crusaded for simplified spelling and the metric system.
dewlap (n.): mid-14c., dewe lappe, from lappe "loose piece" (Old English læppa), first element of unknown origin or meaning. Originally of cattle.
dewpoint (n.): 1833; see dew + point (n.).
dewy (adj.): Old English deawig (see dew + -y (2)).
Dexedrine (n.): 1942, trademark (Smith, Kline and French Laboratories) for dexamphetamine sulphate, probably from dextro- + chemical ending from Benzedrine, etc.
Dexter: masc. proper name, from Latin dexter "on the right hand" (see dexterity). Compare also Benjamin.
dexterity (n.): 1520s, from Middle French dexterité (16c.), from Latin dexteritatem (nominative dexteritas) "readiness, skillfulness, prosperity," from dexter "skillful," also "right (hand)" (source of Old French destre, Spanish diestro, etc.), from PIE root *deks- "on the right hand," hence "south" to one facing east (source also of Sanskrit daksinah "on the right hand, southern, skillful;" Avestan dashina- "on the right hand;" Greek dexios "on the right hand," also "fortunate, clever;" Old Irish dess "on the right hand, southern;" Welsh deheu; Gaulish Dexsiva, name of a goddess of fortune; Gothic taihswa; Lithuanian desinas; Old Church Slavonic desnu, Russian desnoj). The Latin form is with the comparative suffix -ter, thus meaning etymologically "the better direction." Middle English dester meant "right hand," and in heraldry dexter means "on the right side."
dexterous (adj.): c. 1600, "convenient, suitable," formed in English from Latin dexter (see dexterity) + -ous. Meaning "skillful, clever" is from 1620s.
dextro-: word-forming element meaning "toward or on the right-hand side," from comb. form of Latin dexter (see dexterity).
dextrose (n.): 1867, shortened from dextro-glucose, from dextro- "right" + -ose (2), chemical suffix indicating a sugar. So called because this form of glucose polarizes light to the right in spectroscopy.
dextrous (adj.): 1620s, alternative spelling of dexterous; this version is more conformable to Latin but less common in English.
dey (n.1): Old English dæge "female servant, housekeeper, maid," from Proto-Germanic *daigjon (source also of Old Norse deigja "maid, female servant," Swedish deja "dairymaid"), from PIE *dheigh- "to form, build" (see dough). Now obsolete (though OED says, "Still in living use in parts of Scotland"), it forms the first element of dairy and the second of lady.

The ground sense seems to be "kneader, maker of bread;" advancing by Old Norse deigja and Middle English daie to mean "female servant, woman employed in a house or on a farm." Dæge as "servant" is the second element in many surnames ending in -day (such as Faraday, and perhaps Doubleday "servant of the Twin," etc.).
dey (n.2): 1650s, "title of a military commander in Muslim north Africa," from Turkish dai "maternal uncle," a friendly title used of older men, especially by the Janissaries of Algiers of their commanding officers. There were also deys in Tunis and Tripoli.
dharma (n.): 1796, in secular sense, "caste custom, right behavior;" in Buddhism and Hinduism, "moral law," from Sanskrit, "law, right, justice," related to dharayati "holds," and cognate with Latin firmus "strong; stable," figuratively "constant, trusty;" all from PIE root *dher- (2) "to hold firmly, support" (see firm (adj.)).
dhoti: 1620s, from Hindi dhoti.
dhow (n.): 1799, original language unknown, "single-masted native vessel used on Arabian Sea," later widely applied to all Arab vessels. Klein suggests a relation to Persian dav "running."
di- (1): word-forming element meaning "two, double, twice," from Greek di-, from dis "twice," related to duo (see two).
di- (2): word-forming element meaning "apart, asunder," form of dis- before certain voiced consonants.
di- (3): word-forming element meaning "through; thoroughly," form of dia- before vowels.
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."
diabetes (n.): 1560s, from medical Latin diabetes, from late Greek diabetes "excessive discharge of urine" (so named by Aretaeus the Cappadocian, physician of Alexandria, 2c.), literally "a passer-through, siphon," from diabainein "to pass through," from dia- "through" (see dia-) + bainein "to go" (see come).

An old common native name for it was pissing evil. In classical Greek, diabainein meant "to stand or walk with the legs apart," and diabetes meant "a drafting compass," from the position of the legs.
diabetic (adj.): 1799; see diabetes + -ic. From 1840 as a noun.
diabolic (adj.): late 14c., from Old French diabolique (13c.), from Late Latin diabolicus, from Ecclesiastical Greek diabolikos "devilish," from diabolos (see devil (n.)).
diabolical (adj.): c. 1500, "pertaining to the devil," from diabolic + -al (1). Meaning "befitting the devil" is from 1540s. Related: Diabolically.
diabolism (n.): c. 1600s, from Ecclesiastical Greek diabolos "devil" (see devil (n.)) + -ism.
diachronic (adj.): 1857, from Greek dia "throughout" (see dia-) + khronos "time" (see chrono-). Use in linguistics dates from 1927.
diacritic (adj.): 1690s, of sounds, from Greek diakritikos "that separates or distinguishes," from diakrinein "to separate one from another," from dia- (see dia-) + krinein "to separate, decide, judge" (see crisis). As a noun, from 1866. Related: Diacritical.
diadem (n.): late 13c., from Old French diademe and directly from Latin diadema "cloth band worn around the head as a sign of royalty," from Greek diadema, from diadein "to bind across," from dia- "across" (see dia-) + dein "to bind," related to desmos "band," from PIE *de- "to bind." Used of the headband worn by Persian kings and adopted by Alexander the Great and his successors.
diagnose (v.): 1861, back-formation from diagnosis. Related: Diagnosed; diagnosing.
diagnoses (n.): plural of diagnosis.
diagnosis (n.): 1680s, medical Latin application of Greek diagnosis "a discerning, distinguishing," from stem of diagignoskein "discern, distinguish," literally "to know thoroughly," from dia- "apart" (see dia-) + gignoskein "to learn" (see gnostic).
diagnostic: 1620s (adjective and noun), from Greek diagnostikos "able to distinguish," from diagnostos, verbal adjective from diagignoskein (see diagnosis). Related: Diagnostics.
diagonal (adj.): 1540s (implied in diagonally), from Middle French diagonal, from Latin diagonalis, from diagonus "slanting line," from Greek diagonios "from angle to angle," from dia- "across" (see dia-) + gonia "angle" (see -gon). As a noun, from 1570s.
diagram (n.): 1610s, from French diagramme, from Latin diagramma, from Greek diagramma "geometric figure, that which is marked out by lines," from diagraphein "mark out by lines, delineate," from dia- "across, out" (see dia-) + graphein "write, mark, draw" (see -graphy). The verb is 1840, from the noun.
dial (n.): early 15c., "sundial," earlier "dial of a compass" (mid-14c.), apparently from Medieval Latin dialis "daily," from Latin dies "day" (see diurnal).

The word perhaps was abstracted from a phrase such as Medieval Latin rota dialis "daily wheel," and evolved to mean any round plate over which something rotates. Telephone sense is from 1879, which led to dial tone (1921), "the signal to begin dialing," which term soon might be the sole relic of the rotary phone.
dial (v.): 1650s, "to work with aid of a dial or compass," from dial (n.). Telephone sense is from 1923. Related: Dialed; dialing.
dialect (n.): 1570s, "form of speech of a region or group," from Middle French dialecte, from Latin dialectus "local language, way of speaking, conversation," from Greek dialektos "talk, conversation, speech;" also "the language of a country, dialect," from dialegesthai "converse with each other," from dia- "across, between" (see dia-) + legein "speak" (see lecture (n.)).
dialectal (adj.): 1831, from dialect + -al (1).
dialectic (n.): 1580s, earlier dialatik (late 14c.), from Old French dialectique (12c.), from Latin dialectica, from Greek dialektike (techne) "(art of) philosophical discussion or discourse," fem. of dialektikos "of conversation, discourse," from dialektos "discourse, conversation" (see dialect). Originally synonymous with logic; in modern philosophy refined by Kant, then by Hegel, who made it mean "process of resolving or merging contradictions in character." Related: Dialectics.
dialectical (adj.): "argumentative," 1540s; see dialectic + -al (1).
dialog: see dialogue.
dialogue (n.): early 13c., "literary work consisting of a conversation between two or more persons," from Old French dialoge, from Latin dialogus, from Greek dialogos "conversation, dialogue," related to dialogesthai "converse," from dia- "across" (see dia-) + legein "speak" (see lecture (n.)).

Sense broadened to "a conversation" c. 1400. Mistaken belief that it can only mean "conversation between two persons" is from confusion of dia- and di- (1); the error goes back to at least 1532, when trialogue was coined needlessly for "a conversation between three persons." A word that has been used for "conversation between two persons" is the hybrid duologue (1864).
dialysis (n.): 1580s, from Latin, from Greek dialysis "dissolution, separation" (of the disbanding of troops, a divorce, etc.), from dialyein "dissolve, separate," from dia- "apart" + lyein "loosen" (see lose). Used originally in logic and grammar; chemistry sense is first recorded 1861, medicine 1914. Related: Dialytic.
diamante (adj.): 1904, from French diamanté, past participle of diamanter "to set with diamonds," from Old French diamant (see diamond).
diameter (n.): late 14c., from Old French diametre, from Latin diametrus, from Greek diametros (gramme) "diagonal of a circle," from dia- "across, through" (see dia-) + metron "a measure" (see meter (n.2)).