Etymology
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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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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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unnecessary (adj.)
1540s, from un- (1) "not" + necessary (adj.). Related: Unnecessarily.
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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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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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*ked- 
Proto-Indo-European root meaning "to go, yield."

It forms all or part of: abscess; accede; access; ancestor; antecede; antecedent; cease; cede; cession; concede; decease; exceed; excess; incessant; intercede; necessary; precede; predecessor; proceed; recede; recess; recession; secede; secession; succeed; success.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit sedhati "to drive, chase away;" Avestan apa-had- "turn aside, step aside;" Latin cedere "to yield, give place; to give up some right or property," originally "to go from, proceed, leave;" Old Church Slavonic chodu "a walking, going," choditi "to go."
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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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disqualify (v.)

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

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necessitation (n.)

"act of making necessary," 1650s, noun of action from necessitate.

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overeducated (adj.)

also over-educated, "educated to excess or too long or beyond what is necessary," 1788, from over- + educated.

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