default (n.) Look up default at Dictionary.com
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.) Look up default at Dictionary.com
late 14c., "be lacking, be missing," also "become weak," from default (n.). Related: Defaulted; defaulting.
defeasance (n.) Look up defeasance at Dictionary.com
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.
defeat (v.) Look up defeat at Dictionary.com
late 14c., from Anglo-French defeter, from Old French desfait, past participle of desfaire "to undo," from Vulgar Latin *diffacere "undo, destroy," from Latin dis- "un-, not" (see dis-) + facere "to do, perform" (see factitious). Original sense was of "bring ruination, cause destruction." Military sense of "conquer" is c.1600. Related: Defeated; defeating.
defeat (n.) Look up defeat at Dictionary.com
1590s, from defeat (v.).
defeatism (n.) Look up defeatism at Dictionary.com
1918; see defeatist.
defeatist Look up defeatist at Dictionary.com
1918, adjective and noun, in reference to pacifists and political opposition in Britain, from French défaitiste, which was used there in reference to the Russians who sought to end their war with Germany; see defeat (n.) + -ist. Their opposition, in the original Russian context, were called defensists.
defecate (v.) Look up defecate at Dictionary.com
1570s, "to purify," from Latin defaecatus, past participle of defaecare "cleanse from dregs, purify," from the phrase de faece "from dregs" (plural faeces; see feces). Excretory sense first recorded 1830 (defecation), American English, from French. Related: Defecated; defecating.
defecation (n.) Look up defecation at Dictionary.com
1620s, from Late Latin defecationem (nominative deficatio), noun of action from past participle stem of Latin defecare (see defecate). An Old English word for "bowel movement" was arse-gang literally "arse-going."
defect (n.) Look up defect at Dictionary.com
early 15c., from Middle French defect and directly from Latin defectus "failure, revolt, falling away," noun use of past participle of deficere "to fail, desert" (see deficient).
defect (v.) Look up defect at Dictionary.com
1570s, from Latin defectus, past participle of deficere "to fail, desert" (see defect (n.)). Related: Defected; defecting.
defection (n.) Look up defection at Dictionary.com
1540s, "action of failing;" 1550s, "action of deserting a party, leader, etc." from Latin defectionem (nominative defectio) "desertion, revolt, failure," noun of action from past participle stem of deficere (see deficient). Originally used often of faith.
defective (adj.) Look up defective at Dictionary.com
mid-14c., from Middle French défectif (14c.) and directly from Late Latin defectivus, from defect-, past participle stem of deficere (see deficient). A euphemism for "mentally ill" from 1898 to c.1935. Related: Defectively; defectiveness.
defector (n.) Look up defector at Dictionary.com
1660s, agent noun in Latin form from defect, or else from Latin defector "revolter," agent noun from deficere (see deficient).
defence Look up defence at Dictionary.com
see defense.
defend (v.) Look up defend at Dictionary.com
mid-13c., from Old French defendre (12c.) "defend, resist," and directly from Latin defendere "ward off, protect, guard, allege in defense," from de- "from, away" (see de-) + -fendere "to strike, push," from PIE root *gwhen- "to strike, kill" (see bane). In the Mercian hymns, Latin defendet is glossed by Old English gescildeð. Related: Defended; defending.
defendant (n.) Look up defendant at Dictionary.com
c.1400, in the legal sense, from French défendant, present participle of défendre (see defend). Earliest use in English was as a present participle adjective meaning "defending" (c.1300).
defender (n.) Look up defender at Dictionary.com
c.1300 (early 13c. as a surname), via Anglo-French, from Old French defendeor, agent noun from defendre (see defend). The Latin word in this sense was defensor.
defenestration (n.) Look up defenestration at Dictionary.com
1620, "the action of throwing out of a window," from Latin fenestra "window" (see fenestration). A word invented for one incident: the "Defenestration of Prague," May 21, 1618, when two Catholic deputies to the Bohemian national assembly and a secretary were tossed out the window (into a moat) of the castle of Hradshin by Protestant radicals. It marked the start of the Thirty Years War. Some linguists link fenestra with Greek verb phainein "to show;" others see in it an Etruscan borrowing, based on the suffix -(s)tra, as in Latin loan-words aplustre "the carved stern of a ship with its ornaments," genista "the plant broom," lanista "trainer of gladiators." Related: Defenestrate (1915); defenestrated (1620).
defense (n.) Look up defense at Dictionary.com
c.1300, "forbidding, prohibition," also "action of guarding or protecting," from Old French defense, from Latin defensus, past participle of defendere "ward off, protect" (see defend). But it also arrived (without the final -e) from Old French defens, from Latin defensum "thing protected or forbidden," neuter past participle of defendere.

Defens was assimilated into defense, but not before it inspired the alternative spelling defence, via the same tendency that produced hence (hennis), pence (penies), dunce (Duns). First used 1935 as a euphemism for "national military resources." Defense mechanism in psychology is from 1913.
defenseless (adj.) Look up defenseless at Dictionary.com
also defenceless, 1520s, from defense + -less. Related: Defenselessly.
defensible (adj.) Look up defensible at Dictionary.com
late 13c., from Old French defensable, from Late Latin defensibilem, from Latin defens-, past participle stem of defendere (see defend).
defensive Look up defensive at Dictionary.com
c.1400 (adj. and noun), from French défensif (14c.), from Medieval Latin defensivus, from defens-, past participle stem of Latin defendere (see defend). Of persons, "alert to reject criticism," from 1919. Related: Defensively; defensiveness.
defer (v.1) Look up defer at Dictionary.com
"to delay," late 14c., differren, deferren, from Old French differer (14c.), from Latin differre "carry apart, scatter, disperse;" also "be different, differ;" also "defer, put off, postpone," (see differ). Etymologically identical with differ; the spelling and pronunciation differentiated from 15c., perhaps partly by association of this word with delay.
defer (v.2) Look up defer at Dictionary.com
"yield," mid-15c., from Middle French déférer (14c.) "to yield, comply," from Latin deferre "carry away, transfer, grant," from de- "down, away" (see de-) + ferre "carry" (see infer). Main modern sense is from meaning "refer (a matter) to someone," which also was in Latin.
deference (n.) Look up deference at Dictionary.com
1640s, from French déférence (16c.), from déférer (see defer (v.2)).
deferent (adj.) Look up deferent at Dictionary.com
1620s, from French déférent (16c.), from Latin deferentem (nominative deferens), present participle of deferre "to carry down or away" (see defer (v.2)). Earlier in Middle English as a word in astronomy (early 15c.).
deferential (adj.) Look up deferential at Dictionary.com
1822, from deferent + -ial; as a word in anatomy, from 1877. Related: Deferentially.
deferment (n.) Look up deferment at Dictionary.com
1610s, from defer (v.1) + -ment. As a word for "conditional exemption from a military draft" it dates to 1918, American English.
deferral (n.) Look up deferral at Dictionary.com
1895, from defer (v.1) + -al (2).
deferred (adj.) Look up deferred at Dictionary.com
"delayed," 1660s, past participle adjective from defer (v.1).
defiance (n.) Look up defiance at Dictionary.com
c.1300, from Old French desfiance "challenge, declaration of war," from desfiant, present participle of desfier (see defy).
defiant (adj.) Look up defiant at Dictionary.com
1837, from French défiant, present participle of défier (see defy). Related: Defiantly.
defibrillation (n.) Look up defibrillation at Dictionary.com
1940, in reference to heartbeat, from de- + fibrillation "a beating in an abnormal way," from Modern Latin fibrilla, diminutive of fibra "fiber," in reference to the muscle strands of the heart that contract irregularly in this condition.
defibrillator (n.) Look up defibrillator at Dictionary.com
1956, agent noun from defibrillation.
deficiency (n.) Look up deficiency at Dictionary.com
1630s, from deficience (mid-15c.) + -cy; or from Late Latin deficientia, from deficientem (see deficient).
deficient (adj.) Look up deficient at Dictionary.com
1580s, from Latin deficientem (nominative deficiens), present participle of deficere "to desert, revolt, fail," from de- "down, away" (see de-) + facere "to do, perform" (see factitious).
deficit (n.) Look up deficit at Dictionary.com
1782, from French déficit (late 17c.), from Latin deficit "it is wanting," an introductory word in clauses of inventory, third person singular present indicative of deficere "to be deficient" (see deficient).
defilade (n.) Look up defilade at Dictionary.com
1828, from defile (n.) + -ade.
defile (v.) Look up defile at Dictionary.com
c.1400, "to desecrate, profane;" mid-15c., "to make foul or dirty," alteration of earlier defoulen, from Old French defouler "trample down, violate," also "ill-treat, dishonor," from de- "down" (see de-) + foler "to tread," from Latin fullo "person who cleans and thickens cloth by stamping on it" (see foil (v.)).

The alteration (or re-formation) in English is from influence of Middle English filen (v.) "to render foul; make unclean or impure," literal and figurative, from Old English fylen (trans.), related to Old English fulian (intrans.) "to become foul, rot," from the source of foul (adj.). Compare befoul, which also had a parallel form befilen. Related: Defiled; defiling.
defile (n.) Look up defile at Dictionary.com
"narrow passage," 1640s, especially in a military sense, "a narrow passage down which troops can march only in single file," from French défilé, noun use of past participle of défiler "march by files" (17c.), from de- "off" (see de-) + file "row," from Latin filum "thread" (see file (v.)). The verb in this sense is 1705, from French défiler.
defilement (n.) Look up defilement at Dictionary.com
1570s, from defile (v.) + -ment.
define (v.) Look up define at Dictionary.com
late 14c., "to specify; to end," from Old French defenir "to end, terminate, determine," and directly from Latin definire "to limit, determine, explain," from de- "completely" (see de-) + finire "to bound, limit," from finis "boundary, end" (see finish (n.)). Related: Defined; defining.
definite (adj.) Look up definite at Dictionary.com
1550s, from Latin definitus "defined, bounded, limited," past participle of definire (see define). Definite means "defined, clear, precise, unmistakable;" definitive means "having the character of finality."
definitely (adv.) Look up definitely at Dictionary.com
1580s, from definite + -ly (2). As a colloquial emphatic word, attested by 1931.
definition (n.) Look up definition at Dictionary.com
late 14c., "decision, setting of boundaries," from Old French definicion, from Latin definitionem (nominative definitio), noun of action from past participle stem of definire (see define).

In logic, meaning "act of stating what something means" is from 1640s; meaning "a statement of the essential nature of something" is from late 14c.; the special focus on words developed after c.1550. Meaning "degree of distinctness of the details in a picture" is from 1889.
definitive (adj.) Look up definitive at Dictionary.com
late 14c., from Old French definitif (12c.), from Latin definitivus "explanatory, definitive," from past participle stem of definire (see define). Related: Definitively.
deflagration (n.) Look up deflagration at Dictionary.com
c.1600, from Latin deflagrationem (nominative deflagratio) "a burning up, conflagration," noun of action from past participle stem of deflagrare, from de- (see de-) + flagrare (see flagrant).
deflate (v.) Look up deflate at Dictionary.com
1891, in reference to balloons, coinage based on inflate. Latin deflare meant "to blow away," but in the modern word the prefix is taken in the sense of "down." Related: Deflated; deflating.
deflation (n.) Look up deflation at Dictionary.com
1891, "release of air," from deflate + -ion. In reference to currency or economic situations, from 1920.