"Wax and wane" is an idiomatic expression that describes something that increases and decreases in size, strength, or intensity over time. It is often used in reference to the phases of the moon, where "waxing" means it is growing larger and "waning" means it is getting smaller. The phrase can also apply to various situations, such as trends, moods, or fortunes that fluctuate over periods.
The term "wax" in this context comes from Old English weaxan, meaning "to grow," while "wane" comes from the Old English wanian, meaning "to decrease." The phrase combines these two concepts to depict a cyclical pattern of growth and decline.