'antonymous' originates from Modern English, specifically formed from the noun 'antonym' plus the adjectival suffix '-ous' (from Latin/French suffixes meaning 'full of' or 'having the quality of').

'antonymous' traces back to the noun 'antonym,' which itself comes from Greek 'antōnymon' (from anti- 'against' + onoma 'name'); the element was borrowed into New/Modern English as 'antonym' and then extended with '-ous' to form 'antonymous'.

Initially formed to indicate 'having an opposite name/term,' its meaning shifted to the broader modern sense of 'having an opposite meaning' or 'being opposite in meaning.'