Volunteer
late 16th century (as a noun, with military reference): from French volontaire ‘voluntary’. The change in the ending was due to association with -eer.
wiktionary
Borrowed from Middle French voluntaire, from Latin voluntārius(“willing, voluntary”); or from voluntary + -eer.
etymonline
volunteer (n.)
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.
volunteer (v.)
1755, from volunteer (n.). Related: Volunteered; volunteering (1690s as a verbal noun).
