'synonymy' originates from New Latin (or Late Latin), specifically the word 'synonymia', ultimately from Greek 'synonymia', where 'syn-' meant 'together' and 'onoma' (or 'onym-') meant 'name'.

'synonymia' came into Medieval/Modern Latin and Middle English as forms like 'synonymie' and eventually became the modern English word 'synonymy'.

Initially it referred to 'sameness of name' or 'same name', but over time it evolved to mean 'sameness or near-sameness of meaning' in modern usage.