'antimedievally' originates from Modern English, specifically the combining of the prefix 'anti-' (from Greek 'antí') meaning 'against' with 'medieval' (from Latin 'medium' + 'aevum') where 'medium' meant 'middle' and 'aevum' meant 'age'.

'medieval' developed from Latin 'medium aevum' into Neo-Latin 'mediaevalis' and then into Middle/Modern English as 'medieval'; the prefix 'anti-' comes from Greek 'antí' and entered English via Latin and Old French. In Modern English the compound 'antimedieval' was formed, and the adverbial suffix '-ly' produced 'antimedievally'.

Initially, the components meant 'against' and 'middle age'; when combined in Modern English they came to mean 'in a manner opposed to medieval ideas'—a transparent compound rather than a word with a long independent historical sense.