/ ˈlevn; NAmE / noun , verb
■ noun
[ U ] a substance, especially yeast , that is added to bread before it is cooked to make it rise :
( figurative )
A few jokes add leaven to a boring speech.
■ verb
[ vn ] [ often passive ] leaven sth (with sth) ( formal ) to make sth more interesting or cheerful by adding sth to it :
Her speech was leavened with a touch of humour.
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WORD ORIGIN
Middle English : from Old French levain , based on Latin levamen relief (literally means of raising ), from levare to lift.