Definition: Antonomasia is a grammatical construction that refers to a part or phrase being taken out of context without changing its meaning, often used in negation. An antonym of 'antonomatic' means something not antonyms, but it's considered an instance where you can't find a proper synonym for the original word. Definition: Antonomasia is a form of grammatical construction that refers to a part or phrase being taken out of context without changing its meaning, often used in negation. It's considered an instance where you can't find a proper synonym for the original word.