indispensable

Etymology

From Middle French indispensable, from Medieval Latin indispensābilis, corresponding to in- + dispensable.

adj

1. (ecclesiastical, obsolete) Not admitting ecclesiastical dispensation; not subject to release or exemption; that cannot be allowed by bending the canonical rules.
2. (of duties, rules etc.) Unbendable, that cannot be set aside or ignored.
3. Absolutely necessary or requisite; that one cannot do without.
   An indispensable component of a heart-healthy diet.
   But through the oligopoly, charcoal fuel proliferated throughout London's trades and industries. By the 1200s, brewers and bakers, tilemakers, glassblowers, pottery producers, and a range of other craftsmen all became hour-to-hour consumers of charcoal. This only magnified the indispensable nature of the oligopolists. 2006, Edwin Black, chapter 2, in Internal Combustion
   And my friends in politics, no-one is remotely indispensable and our brilliant and Darwinian system will produce another leader, equally committed to taking this country forward through tough times. 7 July 2022, Boris Johnson, resignation speech

noun

1. A thing that is not dispensable; a necessity.
2. (in the plural, colloquial, dated) Trousers.