The word 'unionist' is a relatively modern English formation, built upon the older noun 'union', which entered English in the 15th century from Anglo-French and Old French. 'Union' itself originates from the Latin 'unio' (a unity, oneness), derived from 'unus' meaning "one", a root tracing back to the Proto-Indo-European oi-no- (one, single). The suffix '-ist', borrowed via Latin from Greek '-istes', denotes an agent or adherent. Thus, the literal construction "one who adheres to union" first emerged in the early 19th century to describe supporters of trade unions (labour unionists) and was later applied to political advocates for the union of states or territories, most notably in British and American contexts.

Structure: union (act of uniting) + ist (one who practices or believes) union: From Latin 'unio' (oneness, unity), via Old French. Acts as the core noun base meaning "the act or state of being united". ist: From Greek '-istes', via Latin '-ista'. A noun-forming suffix meaning "a person who practices, is concerned with, or adheres to a principle".