indispensable adj.

word-forming element meaning "not, opposite of, without" (also im-, il-, ir- by assimilation of -n- with following consonant, a tendency which began in later Latin), from Latin in- "not," cognate with Greek an-, Old English un-, all from PIE root *ne- "not."

In Old French and Middle English often en-, but most of these forms have not survived in Modern English, and the few that do (enemy, for instance) no longer are felt as negative. The rule of thumb in English has been to use in- with obviously Latin elements, un- with native or nativized ones.

mid-14c., dispensen, "to dispose of, deal or divide out," from Old French dispenser "give out" (13c.), from Latin dispensare "disburse, administer, distribute (by weight)," frequentative of dispendere "pay out," from dis- "out" (see dis-) + pendere "to hang, cause to hang; weigh; pay" (from PIE root *(s)pen- "to draw, stretch, spin").

In Medieval Latin, dispendere was used in the ecclesiastical sense of "grant licence to do what is forbidden or omit what is required" (a power of popes, bishops, etc.), and thus acquired a sense of "grant remission from punishment or exemption from law," hence the use of the English verb in the senses "to do away with" (1570s), "do without" (c. 1600). The older sense is preserved in dispensary. Related: Dispensed; dispensing.