make impure by adding inferior or tainted substances; water down
Many petrol vendors adulterate the petrol with water.
adulterare (falsify, corrupt, commit adultery) (Latin)
when a child becomes adult he loses his purity and innocence. so any thing adulterated is contaminated or has lost its purity. Adults are more impure than children, so adulterate means to make impure dull+the+rate = impure, means to decrease the rate