volunteer (n.)

c. 1600, "one who offers himself for military service," from Middle 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).