'antonomasic' originates from Greek, specifically the word 'antonomasia' (from 'antonomazein'), where 'anti-' meant 'in place of/for' and 'onoma' meant 'name'.

'antonomasia' passed into Late Latin and then into English (via French/Latin usage); the adjective 'antonomasic' (and variant 'antonomastic') was formed from the noun 'antonomasia' in Modern English.

Initially it referred to the noun sense 'the substitution of a name', but over time the form came to be used adjectivally to mean 'relating to or characteristic of that substitution'.