Is it ensure, insure, or assure?
late 14c. (implied in voluntarily), from Latin voluntarius "of one's free will," from voluntas "will," from the ancient accusative singular present participle of velle "to wish" (see will (v.)). Originally of feelings, later also of actions (mid-15c.).
voluntary vol·un·tar·y (vŏl'ən-těr'ē)
Arising from or acting on one's own free will.
Normally controlled by or subject to individual volition, as of respiration.
Capable of making choices; having the faculty of will.