Is it ensure, insure, or assure?
1530s, "subject to dispensation," from Medieval Latin dispensabilis, from dispensare (see dispense). Meaning "that can be done without" is from 1640s. Related: Dispensability.
dispensable dis·pen·sa·ble (dĭ-spěn'sə-bəl)adj. Capable of being dispensed, administered, or distributed. Used of a drug.