Online Etymology Dictionary

decoupage (n.): 1960, from French découpage, literally "the act of cutting out," from decouper "to cut out" (12c., Old French decoper), from de- "out" (see de-) + couper "to cut" (see chop (v.1)).
decouple (v.): c. 1600, from French découpler "to uncouple," from de- (see de-) + coupler (Old French copler; see couple (v.)). Related: Decoupled; decoupling.
decoy (n.): 1610s, perhaps from Dutch kooi "cage," used of a pond surrounded by nets, into which wildfowl were lured for capture, from West Germanic *kaiwa, from Latin cavea "cage." The first element is possibly the Dutch definite article de, mistaken in English as part of the word. But decoy, of unknown origin, was the name of a card game popular c. 1550-1650, and this may have influenced the form of the word.
decoy (v.): 1650s, from decoy (n.). Related: Decoyed; decoying.
decrease (v.): late 14c., from Anglo-French decreiss-, present participle stem of decreistre, Old French descroistre (12c., Modern French décroître), from Latin decrescere "to grow less, diminish," from de- "away from" (see de-) + crescere "to grow" (see crescent). Related: Decreased; decreasing.
decrease (n.): late 14c., "detriment, harm;" early 15c. as "a becoming less or smaller," from Anglo-French decres; see decrease (v.).
decree (n.): early 14c., from Old French decre, variant of decret (12c., Modern French décret), from Latin decretum, neuter of decretus, past participle of decernere "to decree, decide, pronounce a decision," from de- (see de-) + cernere "to separate" (see crisis).
decree (v.): late 14c., from decree (n.). Related: Decreed; decreeing.
decrement (n.): 1620s, from Latin decrementum "diminution," from stem of decrescere (see decrease).
decrepit (adj.): mid-15c., from Middle French décrépit (15c.), from Latin decrepitus "very old, infirm," from de- "down" (see de-) + *crepitus, past participle of crepare "to crack, break" (see raven).
decrepitude (n.): c. 1600, from French décrépitude (14c.), from Latin decrepitus (see decrepit).
decrescendo (n.): 1806, from Italian decrescendo, from Latin decrescere (see decrease (v.)).
decriminalization (n.): 1945, from de- + criminal + -ization. Especially in reference to narcotics since c. 1968.
decriminalize (v.): 1963, "to reform a criminal," back-formation from decriminalization. Meaning "to make legal something that formerly had been illegal" was in use by 1970 (there are isolated instances back to 1867). Related: Decriminalized; decriminalizing.
decry (v.): 1610s, from French decrier (14c.; Old French descrier "cry out, announce"), from de- "down, out" (see de-) + crier "to cry," from Latin quiritare (see cry (v.)). In English, the sense has been colored by the presumption that de- in this word means "down."
decrypt (v.): "to solve a cryptogram," 1936, from de- + cryptogram. Related: Decrypted; decrypting.
decubitus (n.): 1866, Modern Latin, from Latin decumbere "to lie down," from de- (see de-) + cumbere (see succumb).
decuple (adj.): 1610s, from French décuple (late 15c.), from Latin decuplus "tenfold," from decem- "ten" (see ten) + -plus (see plus).
decussate (v.): 1650s, from Latin decussatus, past participle of decussare "to divide crosswise, to cross in the form of an 'X,'" from decussis "the figure 'ten'" (in Roman numerals, represented by X) from decem "ten" (see ten). As an adjective, from 1825.
dedicate (v.): early 15c. (of churches), from Latin dedicatus, past participle of dedicare "consecrate, proclaim, affirm, set apart," from de- "away" (see de-) + dicare "proclaim" (see diction). Dedicated "devoted to one's aims or vocation" is first attested 1944.
dedication (n.): late 14c., "action of dedicating," from Old French dedicacion (14c., Modern French dédication) "consecration of a church or chapel," or directly from Latin dedicationem, noun of action from dedicare (see dedicate). Meaning "the giving of oneself to some purpose" is c. 1600; as an inscription in a book, etc., from 1590s.
deduce (v.): early 15c., from Latin deducere "lead down, derive" (in Medieval Latin, "infer logically"), from de- "down" (see de-) + ducere "to lead" (see duke (n.)). Originally literal; sense of "draw a conclusion from something already known" is first recorded 1520s, from Medieval Latin. Related: Deduced; deducing.
deduct (v.): early 15c., from Latin deductus, past participle of deducere "lead down, bring away;" see deduce, with which it formerly was interchangeable. Technically, deduct refers to taking away portions or amounts; subtract to taking away numbers. Related: Deducted; deducting.
deductible (adj.): 1610s, "that may be deduced," also "that may be deducted;" from Latin deducere (see deduce) + -ible. As a noun, "deductible thing," by 1927.
deduction (n.): early 15c., "action of deducting," from Middle French déduction or directly from Latin deductionem (nominative deductio), noun of action from past participle stem of deducere (see deduce). Meaning "that which is deducted" is from 1540s. As a term in logic, from Late Latin use of deductio as a loan-translation of Greek apagoge.
deductive (adj.): 1640s, from Latin deductivus, from deduct-, past participle stem of deducere "to deduce" (see deduce). Related: Deductively.
deed (n.): Old English dæd "a doing, act, action, transaction, event," from Proto-Germanic *dædis (source also of Old Saxon dad, Old Norse dað, Old Frisian dede, Middle Dutch daet, Dutch daad, Old High German tat, German Tat "deed," Gothic gadeþs "a putting, placing"), from PIE *dhetis (source also of Lithuanian detis "load, burden," Greek thesis "a placing, setting"), from *dhe- "place, put" (see do). Sense of "written legal document" is early 14c. As a verb, 1806, American English Related: Deeded; deeding.
Deely-bobber (n.): "headband with springs carrying ornaments," 1982 trademark name held by Ace Novelty Company. Earlier it had been a patent name for a type of building blocks, manufactured 1969-1973.
deem (v.): Old English deman "to judge, condemn, think, compute," from root of dom (see doom (n.)). Originally "to pronounce judgment" as well as "to form an opinion." The two judges of the Isle of Man were called deemsters in 17c., a title formerly common throughout England and Scotland and preserved in the surname Dempster.
deemed: past tense of deem (q.v.).
deep (adj.): Old English deop "profound, awful, mysterious; serious, solemn; deepness, depth," deope (adv.), from Proto-Germanic *deupaz (source also of Old Saxon diop, Old Frisian diap, Dutch diep, Old High German tiof, German tief, Old Norse djupr, Danish dyb, Swedish djup, Gothic diups "deep"), from PIE *dheub- "deep, hollow" (source also of Lithuanian dubus "deep, hollow, Old Church Slavonic duno "bottom, foundation," Welsh dwfn "deep," Old Irish domun "world," via sense development from "bottom" to "foundation" to "earth" to "world").

Figurative senses were in Old English; extended 16c. to color, sound. Deep pocket "wealth" is from 1951. To go off the deep end "lose control of oneself" is slang first recorded 1921, probably in reference to the deep end of a swimming pool, where a person on the surface can no longer touch bottom. When 3-D films seemed destined to be the next wave and the biggest thing to hit cinema since talkies, they were known as deepies (1953).
deep (n.): Old English deop "deep water," especially the sea, from the source of deep (adj.).
deep six (n.): "place where something is discarded," by 1921 (in phrase give (something) the deep six), originally in motorboating slang, perhaps from earlier underworld noun sense of "the grave" (1929), which is perhaps a reference to the usual grave depth of six feet. But the phrase (in common with mark twain) also figured in the sailing jargon of sounding, for a measure of six fathoms:
As the water deepened under her keel the boyish voice rang out from the chains: "By the mark five--and a quarter less six--by the deep six--and a half seven--by the deep eight--and a quarter eight." ["Learning the Road to Sea," in "Outing" magazine, Feb. 1918]
In general use by 1940s. As a verb from 1953.
deep-freeze (n.): registered trademark (U.S. Patent Office, 1941) of a type of refrigerator; used generically for "cold storage" since 1949.
deep-seated (adj.): 1741, "having its seat far below the surface;" see seat (v.). Figurative use is from 1847.
deepen (v.): c. 1600, from deep (adj.) + -en (1). Related: Deepened; deepening. The earlier verb had been simply deep, from Old English diepan.
deeply (adv.): Old English deoplice (see deep (adj.)), used in both literal and figurative senses.
deer (n.): Old English deor "animal, beast," from Proto-Germanic *deuzam, the general Germanic word for "animal" (as opposed to man), but often restricted to "wild animal" (source also of Old Frisian diar, Dutch dier, Old Norse dyr, Old High German tior, German Tier "animal," Gothic dius "wild animal," also see reindeer), from PIE *dheusom "creature that breathes," from root *dheu- (1) "cloud, breath" (source also of Lithuanian dusti "gasp," dvesti "gasp, perish;" Old Church Slavonic dychati "breathe").

For prehistoric sense development, compare Latin animal from anima "breath"). Sense specialization to a specific animal began in Old English (usual Old English for what we now call a deer was heorot; see hart), common by 15c., now complete. Probably via hunting, deer being the favorite animal of the chase (compare Sanskrit mrga- "wild animal," used especially for "deer"). Deer-lick is first attested 1778, in an American context.
deerskin (n.): late 14c., from deer + skin (n.).
def (adj.): "excellent," first recorded 1979 in African-American vernacular, perhaps a shortened form of definite, or from a Jamaican variant of death.
deface (v.): mid-14c., "to obliterate," from Old French desfacier "mutilate, destroy, disfigure," from des- "away from" (see dis-) + Vulgar Latin *facia (see face (n.)). Weaker sense of "to mar, make ugly" is late 14c. in English. Related: Defaced; defacing.
defacement (n.): 1560s, from deface + -ment.
defalcate (v.): 1530s, "to lop off," from Medieval Latin defalcatus, past participle of defalcare (see defalcation). Modern scientific use dates from 1808.
defalcation (n.): mid-15c., from Medieval Latin defalcationem (nominative defalcatio), noun of action from past participle stem of defalcare, from de- + Latin falx, falcem "sickle, scythe, pruning hook" (see falcate).
defamation (n.): c. 1300, from Old French diffamacion, Medieval Latin deffamation, from Latin diffamationem (nominative diffamatio), noun of action from past participle stem of diffamare (see defame).
defamatory (adj.): 1590s, from Middle French diffamatoire, Medieval Latin diffamatorius "tending to defame," from diffamat-, past participle stem of diffamare (see defame).
defame (v.): c. 1300, from Old French defamer (13c., Modern French diffamer), from Medieval Latin defamare, from Latin diffamare "to spread abroad by ill report, make a scandal of," from dis- suggestive of ruination + fama "a report, rumor" (see fame (n.)). Related: Defamed; defaming.
default (n.): early 13c., "offense, crime, sin," later (late 13c.) "failure, failure to act," from Old French defaute (12c.) "fault, defect, failure, culpability, lack, privation," from Vulgar Latin *defallita "a deficiency or failure," past participle of *defallere, from Latin de- "away" (see de-) + fallere "to deceive, to cheat; to put wrong, to lead astray, cause to be mistaken; to escape notice of, be concealed from" (see fail (v.)). The financial sense is first recorded 1858; the computing sense is from 1966.
default (v.): late 14c., "be lacking, be missing," also "become weak," from default (n.). Related: Defaulted; defaulting.
defeasance (n.): early 15c., from Anglo-French defesaunce, Old French desfaisance "undoing, destruction," from desfaire (Modern French défaire) "to undo, destroy" (see defeat (v.)). Related: Defease; defeasible.