Online Etymology Dictionary

deflect (v.): 1550s, from Latin deflectere "to bend (something) aside or downward," from de- "away" (see de-) + flectere "to bend" (see flexible). Originally transitive, the intransitive sense is first recorded 1640s. Related: Deflected; deflecting.
deflection (n.): also (and with more etymological propriety) deflexion, c.1600, from Latin deflexionem, noun of action from past participle stem of deflectere (see deflect). Both forms were present 17c., but the spelling with -c- has come to predominate.
defloration (n.): late 14c., "culling of the finest passages from books," from Old French desfloracion (14c.), from Latin deflorationem "plucking of flowers," also "taking of (a woman's) virginity," noun of action from past participle stem of deflorare (see deflower). Cf. also anthology. Also used in Middle English with reference to virginity from c.1400.
deflower (v.): late 14c., "deprive (a maiden) of her virginity," also "excerpt the best parts of (a book)," from Old French desflorer (13c., Modern French déflorer) "to deflower (a garden); to take the virginity of," from Late Latin deflorare, from de- (see de-) + flos "flower" (see flora). Notion is "to strip of flowers," hence "to ravish," which is the oldest sense in English.
The French Indians are said not to have deflowered any of our young women they captivated. [James Adair, "The Life of an Indian Trader," London, 1775]
defogger (n.): 1966, from agent noun from de- + fog (v.).
defoliant (adj.): 1943, from defoliate + -ant.
defoliate (v.): 1793, perhaps a back-formation from defoliation. Earlier in this sense was defoil (c.1600). Related: Defoliated; defoliating.
defoliation (n.): 1650s, noun of action from past participle stem of Late Latin defoliare "shed leaves," from de- (see de-) + folium "leaf" (see folio).
deforest (v.): 1880 in modern sense, from de- + forest. Related: Deforested; deforesting. Disforest in the sense "to clear of trees" is from 1660s. Disafforest is attested in this sense from 1842; originally it meant "reduce from the legal status of a forest" (1590s).
deforestation (n.): 1884, from deforest + -ation. Earlier was deforesting (1530s) which was a legal term for the change in definition of a parcel of land from "forest" to something else.
deform (v.): c.1400, "to disfigure," from Old French deformer (13c.), from Latin deformare "put out of shape, disfigure," from de- (see de-) + formare (see form (v.)). Related: Deformed; deforming.
deformation (n.): mid-15c., "transformation," from Old French deformation and directly from Latin deformationem (nominative deformatio), noun of action from past participle stem of deformare (see deform).
deformity (n.): early 15c., diformyte, from Old French deformité "deformity, disfigurement," from Latin deformitatem (nominative deformitas) "ugliness," from deformis "misformed, misshapen," from deformare (see deform).
defragment (v.): 1992, in computer sense, from de- + fragment. Related: Defragmented; defragmenting.
defraud (v.): mid-14c., from Old French defrauder, from Latin defraudare "to defraud, cheat," from de- "thoroughly" (see de-) + fraudare (see fraud). Related: Defrauded; defrauding.
defray (v.): 1540s, from Middle French defraier (15c.), from de- "out" (see de-) + fraier "spend," from Old French frais "costs, damages caused by breakage," from Latin fractum, neuter past participle of frangere "to break" (see fraction). Alternative etymology traces second element to Old High German fridu "peace," via Vulgar Latin *fredum "fine, cost."
defrock (v.): 1580s, from French défroquer (15c.), from de- (see de-) + froque "frock" (see frock). Related: Defrocked.
defrost (v.): 1895, from de- + frost. Related: Defrosted; defrosting.
deft (adj.): Old English gedæfte "mild, gentle," differentiated in Middle English into daft (q.v.) and this word, via sense of "apt, skillful, adept." Cognate with Gothic gadaban "to be fit," Old Norse dafna "to grow strong," Dutch deftig "important, relevant."
deftly (adv.): mid-15c., from deft + -ly (2).
defunct (adj.): 1590s, from Old French defunct (14c., Modern French defunt) or directly from Latin defunctus "dead," literally "off-duty," from past participle of defungi "to discharge, finish," from de- "off, completely" (see de-) + fungi "perform or discharge duty," from PIE root *bheug- (2) "to enjoy" (see brook (v.)).
defuse (v.): 1943, from de- + fuse. Related: Defused; defusing.
defy (v.): c.1300, "to renounce one's allegiance;" mid-14c., "to challenge, defy," from Old French defier, desfier "to challenge, defy, provoke; renounce (a belief), repudiate (a vow, etc.)," from Vulgar Latin *disfidare "renounce one's faith," from Latin dis- "away" (see dis-) + fidus "faithful" (see faith).
degauss (v.): "de-magnetize," originally especially of ships as a defense against magnetic mines, 1940, from German scientist Carl Friedrich Gauss (1777-1855), a pioneer in the study of magnetics.
degeneracy (n.): 1660s, from degenerate + -cy.
degenerate (adj.): late 15c., from Latin degeneratus, past participle of degenerare "to be inferior to one's ancestors, to become unlike one's race or kind, fall from ancestral quality," used of physical as well as moral qualities, from phrase de genere, from de + genus (genitive generis) "birth, descent" (see genus). The noun is from 1550s.
degenerate (v.): 1540s, from Latin degeneratus, past participle of degenerare "fall from ancestral quality" (see degenerate (adj.)). Figurative sense of "to fall off, decline" was in Latin. Related: Degenerated; degenerating.
degeneration (n.): c.1600, from French dégéneration (15c.) or directly from Late Latin degenerationem (nominative degeneratio), noun of action from past participle stem of Latin degenerare (see degenerate (adj.)).
degenerative (adj.): 1846; see degenerate + -ive.
deglutition (n.): from French déglutition (16c.), from Latin deglutitionem, noun of action from past participle stem of deglutare, from de- (see de-) + glutire "to swallow," from PIE *gwele- "to swallow."
degradation (n.): 1530s, from French dégradation (14c., Old French degradacion), from Medieval Latin degradationem (nominative degradatio), noun of action from past participle stem of degradare (see degrade).
degrade (v.): late 14c., from Old French degrader (12c.) "degrade, deprive (of office, rank, etc.)," from des- "down" (see dis-) + Latin gradus "step" (see grade (n.)). Related: Degraded; degrading.
degree (n.): early 13c., from Old French degré (12c.) "a step (of a stair), pace, degree (of relationship), academic degree; rank, status, position," said to be from Vulgar Latin *degradus "a step," from Late Latin degredare, from Latin de- "down" (see de-) + gradus "step" (see grade (n.)).

Most modern senses date from Middle English, from notion of a hierarchy of steps. Meaning "a grade of crime" is 1670s; that of "a unit of temperature" is from 1727. The division of the circle into 360 degrees was known in Babylon and Egypt. It is perhaps from the daily motion of the sun through the zodiac in the course of a year.
degression (n.): late 15c., from Latin degressionem (nominative degressio) "a going down," noun of action from past participle stem of degredi "to go down, march down, descend," from de- "down" (see de-) + gradus "step" (see grade (n.)).
degustation (n.): 1650s, from Latin degustationem (nominative degustatio) "a tasting," noun of action from past participle stem of degustare "to take a taste from, sample," from de- (see de-) + gustare "to taste" (see gusto).
dehiscence (n.): 1828, from Modern Latin dehiscentia, from dehiscentem (nominative dehiscens), present participle of dehiscere "to gape, open, split down" (of the earth, etc.), from de- (see de-) + hiscere, inchoative of hiare "to yawn" (see yawn (v.)).
dehumanize (v.): 1818, from de- + humanize. Related: Dehumanized; dehumanizing.
dehumidifier (n.): 1921, agent noun from de- + humidify.
dehydrate (v.): 1854, from de- + hydrate (v.). A chemical term at first, given a broader extension 1880s. Related: Dehydration (1834).
deicide (n.): 1610s, "the killing of a god;" 1650s, "one who kills a god," from Latin deus "god" (see Zeus) + -cida (see -cide).
deictic (adj.): 1828, from Latinized form of Greek deiktikos "able to show," from deiktos "shown," verbal adjective from deiknynai "to show" (see diction).
deific (adj.): late 15c., from French déifique (late 14c.), from Late Latin deificus "god-making, sacred," in Medieval Latin "divine," from deus "god" (see Zeus) + -ficus "making" (see factitious).
deification (n.): late 14c., from Late Latin deificationem (nominative deificatio), noun of action from past participle stem of deificare (see deify).
deify (v.): mid-14c., from Old French deifier (13c.), from Late Latin deificare, from deificus "making godlike," from Latin deus "god" (see Zeus) + -ficare, from facere "to make, do" (see factitious). Related: Deified; deifying.
deign (v.): c.1300, from Old French deignier (Modern French daigner), from Latin dignari "to deem worthy or fit" (source of Italian degnare, Spanish deñar), from dignus "worthy" (see dignity). Sense of "take or accept graciously" led to that of "condescend" (1580s). Related: Deigned; deigning.
deinstitutionalization (n.): 1967 (disinstitutionalization is attested from 1955), from de- + institutionalization.
deipnosophist (n.): "gourmand," 1650s, from Greek deipnosophistes "one learned in the mysteries of the kitchen," from deipnon "chief meal, dinner" (of unknown origin) + sophistes "master of a craft" (see sophist). the word has come down thanks to "Deipnosophistai," 3c. work on gastronomy by Athenaeus.
deism (n.): 1680s (deist is from 1620s), from French déisme, from Latin deus "god" (see Zeus). Until c.1700, opposed to atheism, in a sense where we now would use theism (see theist).
deist (n.): 1620s, from French déiste, from Latin deus (see Zeus). Related: Deistic (1795). Also cf. deism.
deity (n.): c.1300, "divine nature;" late 14c., "a god," from Old French deité, from Late Latin deitatem (nominative deitas) "divine nature," coined by Augustine from Latin deus "god," from PIE *deiwos (see Zeus).