'fermentative' originates from Latin, specifically the word 'fermentum', where 'fermentum' meant 'leaven' or 'that which causes fermentation'.

'fermentum' passed into Old French as 'ferment' and into Middle English as 'ferment'; the adjectival form 'fermentative' was formed in Modern English by adding the suffix '-ative' (from Latin '-ativus') to the stem 'ferment-'.

Initially, it referred to something related to 'leaven' or an agent that causes fermentation; over time it evolved into the adjectival sense 'relating to or producing fermentation'.