Is it ensure, insure, or assure?
c.1600, "one who offers himself for military service," from Middle French voluntaire, noun use of adj. meaning "voluntary," from Latin voluntarius "voluntary, of one's free will" (see voluntary). Non-military sense is first recorded 1630s. The verb is first recorded 1755, from the noun. Tennessee has been the Volunteer State since the Mexican War, when a call for 2,800 volunteers brought out 30,000 men.