'unleaven' originates from Middle English, formed from the prefix 'un-' (Old English negative) + 'leaven', where 'un-' meant 'not' and 'leaven' meant 'a substance (such as yeast) that makes dough rise.'

'leaven' comes from Old French 'levain', ultimately from Latin 'levare' meaning 'to raise'; the Modern English 'unleaven' developed in Middle English by combining the negative prefix with 'leaven' to mean 'not raised.'

Initially it meant 'not raised (by leaven)', and over time this basic sense has been retained; it also acquired figurative senses of 'plain' or 'unadulterated.'