Meaning

noun (military) a person who freely enlists for service

noun composition (often improvised) for a solo instrument (especially solo organ) and not a regular part of a religious service or musical performance

adjective of your own free will or design; done by choice; not forced or compelled

originFrom Middle English voluntarie, from Old French volontaire, from Latin voluntārius (“willing, of free will”), from voluntās (“will, choice, desire”), from volēns, present participle of volo (“to will”). Displaced native Old English selfwille (literally “self-willed”).

Etymology from Wiktionary, CC BY-SA 3.0