leaven (n.) Look up leaven at Dictionary.com
mid-14c., from Old French levain "leaven, sourdough" (12c.), from Latin levamen "alleviation, mitigation," but used in Vulgar Latin in its literal sense of "a means of lifting, something that raises," from levare "to raise" (see lever). Figurative use from late 14c.
leaven (v.) Look up leaven at Dictionary.com
c.1400, from leaven (n.). Related: Leavened; leavening.