leaven

Etymology

From Middle English levayn, borrowed from Old French levain, from Vulgar Latin *levāmen, a noun based on Latin levō (“raise”).

noun

1. Any agent used to make dough rise or to have a similar effect on baked goods.
2. (figurative) Anything that induces change, especially a corrupting or vitiating change.
   The leaven of insincerity worked through all the fibres of his being. 1926, T.E. Lawrence, Seven Pillars of Wisdom, New York: Anchor, published 1991, page 213

verb

1. (transitive) To add a leavening agent.
2. (transitive) To cause to rise by fermentation.
3. (transitive, figurative) To temper an action or decision.
4. (transitive, figurative) To imbue; to infect; to vitiate.
   With these and the like deceivable doctrines, he levens also his prayer. 1649, John Milton, Eikonoklastes, London, published 1756, page 30
   […] pursue virtue virtuously: leven not good actions, nor render virtues disputable. Stain not fair acts with foul intentions […] 1716, Thomas Browne, edited by Samuel Johnson, Christian Morals, 2nd edition, London: J. Payne, published 1756, Part I, p. 7
5. To rise or become larger.