A Quick and Simple Dictionary

indispensable

1. Adjective.  (ecclesiastical, obsolete) Not admitting ecclesiastical dispensation; not subject to release or exemption; that cannot be allowed by bending the canonical rules. (defdate, 16th-17th c.) .
2. Adjective.  (context, of duties, rules etc.) Unbendable, that cannot be set aside or ignored. (defdate, from 17th c.) .
3. Adjective.  Absolutely necessary or requisite; that one cannot do without. (defdate, from 17th c.) .
4. Noun.  A thing that is not dispensable; a necessity. (defdate, from 17th c.) .
5. Noun.  (context, in plural) Trousers. (defdate, from 19th c.) .