'simpleton' originates from English, specifically from the adjective 'simple' (Middle English 'simple', from Old French 'simple' and Latin 'simplex'), where 'simple' meant 'plain' or 'not compound' and the suffix '-ton' functioned as a diminutive/pejorative forming a noun meaning 'a simple person'.

'simpleton' changed from Middle English formations combining 'simple' + a diminutive/pejorative suffix (recorded in Early Modern English) and eventually became the modern English word 'simpleton'.

Initially, it meant 'a plain or uncomplicated person', but over time it evolved into its current meaning of 'a foolish or gullible person'.