'epistolary' originates from Latin, specifically the word 'epistolaris', where 'epistola' meant 'letter'.

'epistolary' changed from Latin 'epistolaris' (from 'epistola') and entered English via Old French/Medieval Latin forms (e.g. Old French 'epistolaire'), eventually becoming the modern English word 'epistolary'.

Initially it meant 'of or pertaining to letters', and over time it has retained that core meaning while also coming to denote works composed as letters (e.g. an epistolary novel).