'anti-monopolist' is formed from the prefix 'anti-' (from Greek 'antí', meaning 'against') plus 'monopolist' (from 'monopoly' + English agent suffix '-ist').

'monopoly' comes from Greek 'monopōlion' (from monos 'single' + pōlein 'to sell'), passed into Late Latin/Medieval Latin and Old French before entering Middle English as 'monopoly'; the agent-form 'monopolist' developed in Modern English, and the compound 'anti-monopolist' was formed by adding the combining prefix 'anti-'.

Originally the elements meant 'against' + 'one who holds exclusive control of trade'; the combined term has retained the core sense of 'a person opposing exclusive market control' into modern usage.