'unfermentable' originates from English, formed from the prefix 'un-' (Old English negative prefix), the root 'ferment' (from Latin 'fermentum'), and the suffix '-able' (from Latin '-abilis'), where 'fermentum' meant 'leaven' or 'that which causes fermentation'.

'ferment' came from Latin 'fermentum', passed into Old French as 'ferment' and then into Middle English as 'ferment'; the modern English adjective 'unfermentable' was created by combining 'un-' + 'ferment' + '-able' in post-medieval English.

Initially 'fermentum' referred to 'leaven' or a substance that causes bubbling/fermentation; over time the sense broadened to chemical/biological processes of fermentation, and 'unfermentable' now specifically means 'not capable of being fermented'.