As a noun, "indispensable thing," from 1794; c. 1800-1810, after French use, it was the name of a type of pocket bag worn by women. indispensables (1820) also was one of the many 1820s jocular euphemisms for "trousers" (see inexpressible). Related: Indispensably.
1560s, raliabill, "that may be relied on, fit to be depended on, trustworthy," originally Scottish; see rely + -able. Not common before 1850, and often execrated thereafter in Britain as an Americanism because it involves a use of -able different from its use in provable, etc., and not warranted in classical Latin. But it is defended (by OED, Century Dictionary, etc.) on grounds of the suffix's sense in available, laughable, livable, dependable; indispensable, etc. Related: Reliably; reliableness. As a noun, "a reliable person, beast, or thing," by 1890 (with old).
Reliable expresses what cannot be expressed by any other one word. Moreover, it conveys an idea of constant occurrence. He who would be exact has, indeed, no alternative, if he avoids it except a periphrasis. [Fitzedward Hall, "On English Adjectives in -Able," 1877]
Classicism criticises; romanticism creates. Classicism enjoins self-control; romanticism encourages self-indulgence. Classicism is mold; romanticism is matter. Classicism is art; romanticism is nature. Classicism is law; romanticism is life. Romanticism is undoubtedly first and indispensable; but so, not less, classicism is indispensable, though second. [William C. Wilkinson, "Classic French Course in English," 1890]
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").
"that which has to be done, seen, or experienced," 1892, from must (v.). As an adjective, "obligatory, indispensable," by 1912, from the noun; must-read (n.) is from 1959.
"needed, necessary, required by circumstances or the nature of things, so needful that it cannot be dispensed with," mid-15c., from Latin requisitus, past participle of requirere "seek to know, ask, ask for" (see require). As a noun, "that which is necessary, something indispensable," from c. 1600. Related: Requisiteness.
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.).
Ethnology is a very modern science, even later than Geology, and as yet hardly known in America, although much cultivated latterly in Germany and France, being considered an indispensable auxiliary to history and geography. ["Atlantic Journal and Friend of Knowledge," Philadelphia, summer 1832]