c. 1600, "one who offers himself for military service," from French voluntaire, "one who volunteers," also as an adjective, "voluntary," from Latin voluntarius "voluntary, of one's free will," as a plural noun "volunteers" (see voluntary). Non-military sense is first recorded 1630s. As an adjective from 1640s. Tennessee has been the Volunteer State since the Mexican War, when a call for 2,800 volunteers brought out 30,000 men.
1755, from volunteer (n.). Related: Volunteered; volunteering (1690s as a verbal noun).