'unflavoured' is formed by the negative prefix 'un-' (Old English) attached to 'flavoured'. The element 'flavour' (Middle English) comes from Old French (variants like 'flaour'/'flavor') and ultimately from Late Latin 'flāvor' (a formation related to taste/smell), possibly linked to Latin 'flāvus' meaning 'yellow'.

'flavour' passed into Middle English from Old French and developed into the modern English 'flavour'; adding the productive English prefix 'un-' produced the adjective 'unflavoured' meaning 'not flavoured'.

Originally the root 'flavour' referred to scent or taste generally; over time, 'unflavoured' came to specifically mean 'without added flavoring' (especially in food contexts).