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defector (n.)[defector 词源字典]: 1660s, agent noun in Latin form from defect, or else from Latin defector "revolter," agent noun from deficere (see deficient).[defector etymology, defector origin, 英语词源]
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.