The story
From waning + moon. Compare Middle English waynynge of the mone, wanynge of þe mone (literally “waning of the moon”).
Etymology from Wiktionary, CC BY-SA 4.0.
waning moon
Today it means: A moon (usually the Moon, that is, Earth's moon) when it appears smaller each night as it progresses from a full moon to a new moon: a moon in any of the diminishing lunar phases. Full definition →