'leaven' originates from Old French, specifically the word 'levain', where the root derives from Latin 'levare' meaning 'to raise' (from 'levis' meaning 'light').

'levain' in Old French was borrowed into Middle English as forms like 'leven' or 'leuen' and eventually became the modern English word 'leaven'.

Initially it meant 'a substance (such as yeast) that makes dough rise', but over time it acquired the additional figurative meaning 'to enliven or make less heavy'.