'synonymously' originates from Modern English, formed by adding the adverbial suffix '-ly' to the adjective 'synonymous'; 'synonymous' ultimately comes from Greek 'synōnymos' (συνώνυμος), where 'syn-' meant 'together' and 'ónoma' meant 'name'.

'synonymously' was formed in English by converting the adjective 'synonymous' (which itself came into English via Latin/Greek roots: Greek 'synōnymon' → Late Latin/Medieval Latin 'synonymum' → French/English 'synonym') and adding the productive English adverb suffix '-ly', resulting in the modern adverb 'synonymously'.

Initially the Greek-derived elements conveyed 'having the same name' or 'same designation'; over time this developed into the sense 'having the same meaning', and in modern English 'synonymously' means 'in a manner that expresses the same or very similar meaning'.