Etymology
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necessary (n.)

also necessaries, mid-14c., "that which is indispensable; needed, required, or useful things; the necessities of life; actions determined by right or law; that which cannot be disregarded or omitted," perhaps from Old French necessaire (n.) "private parts, genitalia; lavatory," and directly from Latin necessarius (n.), in classical Latin "a relation, relative, kinsman; friend, client, patron;" see necessary (adj.).

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condition (v.)

late 15c., "to make conditions, stipulate," from condition (n.). Meaning "subject to something as a condition" is from 1520s; sense of "form a prerequisite of" is from 1868. Meaning "to bring to a desired condition" is from 1844; psychological sense of "teach or accustom (a person or animal) to certain habits or responses" is from 1909. Related: Conditioned; conditioning.

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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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condition (n.)

mid-14c., condicioun, "particular mode of being of a person or thing," also "a requisite or prerequisite, a stipulation," from Old French condicion "stipulation; state; behavior; social status" (12c., Modern French condition), from Medieval Latin conditionem (nominative conditio), properly condicio "agreement; stipulation; the external position, situation, rank, place, circumstances" of persons, "situation, condition, nature, manner" of things, from condicere "to speak with, talk together, agree upon," in Late Latin "consent, assent," from assimilated form of com "together" (see con-) + dicere "to speak" (from PIE root *deik- "to show," also "pronounce solemnly").

Classical Latin condicio was confused in Late Latin with conditio "a making," from conditus, past participle of condere "to put together." The sense evolution in Latin apparently was from "stipulation" to "situation, mode of being."

Meaning "rank or state with respect to ordered society" is from late 14c. in English. From the notion of "prerequisite" comes the sense of "a restricting or limiting circumstance" (late 14c.). Also in Middle English "personal character, disposition" (mid-14c.).

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prerequisite 

1630s (n.) "a thing or condition required beforehand," 1650s (adj.), "required beforehand, necessary as a condition of something following;" see pre- "before" + requisite. A verb prerequire "require beforehand" is attested from 1610s.

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redundancy (n.)

c. 1600, "condition of superfluity, overabundance;" see redundant + -ancy. The meaning "that which is redundant" is by 1630s; the sense in employment ("superfluous jobs, jobs in excess of what are necessary") is by 1931, chiefly British.

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necessarily (adv.)

late 14c., necesserili, "inevitably, unavoidably, so that it cannot be otherwise," from necessary (adj.) + -ly (2). As "a necessary result or consequence" from c. 1500.

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needful (adj.)

c. 1200, niedfulle, "necessary, needed, useful," also "in want, poor, hungry, starving, having or exhibiting need or distress," from need (n.) + -ful. Meaning "characterized by need" is from mid-13c. From mid-14c. as "indispensable, necessary," also "urgent, demanding attention."

As a noun, "the poor," from c. 1200. The meaning "what is necessary" is from 1709. The colloquial sense of "cash" is recorded from 1774 in phrase the needful "ready money." Related: Needfully; needfulness.

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necessity (n.)

late 14c., necessite, "constraining power of circumstances; compulsion (physical or moral), the opposite of liberty; a condition requisite for the attainment of any purpose," from Old French necessité "need, necessity; privation, poverty; distress, torment; obligation, duty" (12c.), from Latin necessitatem (nominative necessitas) "compulsion, need for attention, unavoidableness, destiny," from necesse (see necessary). Meaning "condition of being in need, want of the means of living" in English is from late 14c.

Necessity is the Mother of Invention. [Richard Franck, c. 1624-1708, English author and angler, "Northern Memoirs," 1658]

To maken vertu of necessite is in Chaucer. Related: Necessities.

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disqualify (v.)

"deprive of necessary disqualifications," 1718 (implied in disqualified), from dis- + qualify. Related: Disqualifying.

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