'antimonopolist' originates from Modern English, formed by the prefix 'anti-' (from Greek 'anti-' meaning 'against') attached to 'monopolist', itself from 'monopoly'.

'antimonopolist' developed as a compound from 'anti-' + 'monopolist'. 'monopolist' comes from 'monopoly' (Middle English < Old French/Latin < Greek 'monopōlion' from 'monos' 'single' + 'polein' 'to sell'). Over time these elements combined in English to form 'antimonopolist'.

Initially it literally meant 'one who is against monopoly'; the core meaning has remained, though usage has broadened to include advocates of antitrust policy and broader anti-monopoly movements.