Online Etymology Dictionary

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.
defeat (v.): 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.): 1590s, from defeat (v.).
defeatism (n.): 1918; see defeatist.
defeatist: 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.): 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.): 1620s, from Late Latin defecationem (nominative deficatio), from defecat-, past participle stem of Latin defecare (see defecate). An Old English word for "bowel movement" was arse-gang literally "arse-going."
defect (n.): 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.): 1570s, from Latin defectus, past participle of deficere "to fail, desert" (see defect (n.)). Related: Defected; defecting.
defection (n.): 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.): 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.): 1660s, agent noun in Latin form from defect, or else from Latin defector "revolter," agent noun from deficere (see deficient).
defence: see defense.
defend (v.): 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.): 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.): 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.): 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.): 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.): also defenceless, 1520s, from defense + -less. Related: Defenselessly.
defensible (adj.): late 13c., from Old French defensable, from Late Latin defensibilem, from Latin defens-, past participle stem of defendere (see defend).
defensive: 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): "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): "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.): 1640s, from French déférence (16c.), from déférer (see defer (v.2)).
deferent (adj.): 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.): 1822, from deferent + -ial; as a word in anatomy, from 1877. Related: Deferentially.
deferment (n.): 1610s, from defer (v.1) + -ment. As a word for "conditional exemption from a military draft" it dates to 1918, American English.
deferral (n.): 1895, from defer (v.1) + -al (2).
deferred (adj.): "delayed," 1660s, past participle adjective from defer (v.1).
defiance (n.): c.1300, from Old French desfiance "challenge, declaration of war," from desfiant, present participle of desfier (see defy).
defiant (adj.): 1837, from French défiant, present participle of défier (see defy). Related: Defiantly.
defibrillation (n.): 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.): 1956, agent noun from defibrillation.
deficiency (n.): 1630s, from deficience (mid-15c.) + -cy; or from Late Latin deficientia, from deficientem (see deficient).
deficient (adj.): 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.): 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.): 1828, from defile (n.) + -ade.
defile (v.): 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.). Cf. befoul, which also had a parallel form befilen. Related: Defiled; defiling.
defile (n.): "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.): 1570s, from defile (v.) + -ment.
define (v.): 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.): 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.): 1580s, from definite + -ly (2). As a colloquial emphatic word, attested by 1931.
definition (n.): 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.): late 14c., from Old French definitif (12c.), from Latin definitivus "explanatory, definitive," from past participle stem of definire (see define). Related: Definitively.
deflagration (n.): 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.): 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.): 1891, "release of air," from deflate + -ion. In reference to currency or economic situations, from 1920.