Perhaps because these once-popular words are now used infrequently, they are often misunderstood or confused. Wax means "to grow gradually larger," "to increase in strength or size." Wane means the exact opposite: "to decrease," "to decline." Both terms are most often used to refer to the comparative fullness of the moon, but they can and do appear in statements such as these: "His anger waxed strong and then subsided." "My enthusiasm for your plan is beginning to wane." Words related to wax are grow, extend, enlarge, dilate, and lengthen. Near-synonyms of wane include fail, diminish, and sink.
Dictionary of problem words and expressions. Harry Shaw. 1975.