Etymology
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unavoidable (adj.)

mid-15c., from un- (1) "not" + avoidable. Related: Unavoidably.

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casualty (n.)

early 15c., casuelte, caswelte, "chance, accident; incidental charge," from casual (adj.) on the model of royalty, penalty, etc. From the earliest use especially of untoward events or misfortunes. The meaning "losses in numbers from a military or other troop" is from late 15c. The meaning "an individual killed, wounded, or lost in battle" is from 1844. Casuality had some currency 16c.-17c. in the sense "chance, a chance occurrence," especially an unfortunate one, but now is obsolete.

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inevitable (adj.)

"unavoidable," mid-15c., from Latin inevitabilis "unavoidable," from in- "not, opposite of" (see in- (1)) + evitabilis "avoidable," from evitare "to avoid," from ex "out" (see ex-) + vitare "shun," originally "go out of the way." As a noun from 1850. Related: Inevitableness.

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necessitate (v.)

1620s, "force irresistably, compel, oblige," also "make necessary, render unavoidable," from Medieval Latin necessitatus, past participle of necessitare "to render necessary," from Latin necessitas "compulsion; destiny" (see necessity). Earlier verb in English was necessen (late 14c.). Related: Necessitated; necessitates; necessitating.

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ineluctable (adj.)

"not to be escaped by struggling," 1620s, from French inéluctable (15c.) or directly from Latin ineluctabilis "unavoidable, inevitable," from in- "not, opposite of" (see in- (1)) + eluctabilis "that may be escaped from," from eluctari "to struggle out of," from ex "out, out of" (see ex-) + luctari "to struggle" (see reluctance).

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Bircher (n.)

1961, member of the U.S. anti-communist John Birch Society, which was founded 1958 and named for John Birch, U.S. Baptist missionary and Army Air Forces captain killed by Chinese Communists shortly after the end of World War II, who is considered the first American casualty of the Cold War.

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necessary (adj.)

late 14c., necessarie, "needed, required; essential, indispensable; such as must be, that cannot be otherwise; not voluntary or governed by chance or free will," from Old French necessaire "necessary, urgent, compelling" (13c.), and directly from Latin necessarius "unavoidable, indispensable, necessary," from necesse "unavoidable, indispensable," originally "no backing away," from ne- "not" (from PIE root *ne-) + cedere "to withdraw, go away, yield" (from PIE root *ked- "to go, yield").

The etymological sense is of that from which there is no evasion, that which is inevitable. Necessary house "privy" is from 1610s (compare Medieval Latin necessarium "a privy"). Necessary evil is from 1540s (the original reference was to "woman").

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*kad- 

Proto-Indo-European root meaning "to fall."

It forms all or part of: accident; cadaver; cadence; caducous; cascade; case (n.1); casual; casualty; casuist; casus belli; chance; cheat; chute (n.1); coincide; decadence; decay; deciduous; escheat; incident; occasion; occident; recidivist.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit sad- "to fall down;" Latin casus "a chance, occasion, opportunity; accident, mishap," literally "a falling," cadere "to fall, sink, settle down, decline, perish;" Armenian chacnum "to fall, become low;" perhaps also Middle Irish casar "hail, lightning."

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either 

Old English ægðer, contraction of æghwæðer (pron., adv., conj.) "each of two, both," from a "always" (see aye (adv.)) + ge- collective prefix + hwæðer "which of two, whether" (see whether). Cognate with Old Frisian eider, Dutch ieder, Old High German eogiwedar, German jeder "either, each, every").

Modern sense of "one or the other of two" is late 13c. Adverbially, for emphasis, "in any case, at all," especially when expressing negation, by 1828. Use of either-or to suggest an unavoidable choice between alternatives (1931) in some cases reflects Danish enten-eller, title of an 1843 book by Kierkegaard.

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pun (n.)

"a Conceit arising from the use of two Words that agree in the Sound, but differ in the Sense" [Addison]; "An expression in which the use of a word in two different applications, or the use of two different words pronounced alike or nearly alike, presents an odd or ludicrous idea" [Century Dictionary]; 1660s (first attested in Dryden), a word of uncertain origin.

Perhaps from pundigron, meaning the same thing (though attested first a few years later), itself a word of uncertain etymology, perhaps a humorous alteration of Italian puntiglio "equivocation, trivial objection," diminutive of Latin punctum "point." This is pure speculation. Punnet was another early form.

Pun was prob. one of the clipped words, such as cit, mob, nob, snob, which came into fashionable slang at or after the Restoration. [OED]

 The verb, "to make puns," also is attested from 1660s, first in Dryden. Related: Punned; punning.

At the revival of learning, and the spread of what we may term the refinement of society, punning was one of the few accomplishments at which the fine ladies and gentlemen aimed. From the twelfth to the sixteenth century, it was at its greatest height. The conversation of the witty gallants, and ladies, and even of the clowns and other inferior characters, in the comedies of Shakespeare and his contemporaries, which we may be sure was painted from the life, is full of puns and plays upon words. The unavoidable result of such an excess was a surfeit, and the consequent dégout, which lasted for more than a century. Like other diseases, it broke out again subsequently with redoubled virulence, and made great havoc in the reign of Queen Anne. [Larwood & Hotten, "The History of Signboards from the Earliest Times to the Present Day," London, 1867] 
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