Old English wanian "make or become smaller gradually, diminish, decline, fade," from Proto-Germanic *wanōnan (source also of Old Saxon wanon, Old Norse vana, Old Frisian wania, Middle Dutch waenen, Old High German wanon "to wane, to grow less"), from *wano- "lacking," from PIE *weno-, suffixed form of root *eue- "to leave, abandon, give out." Related: Waned; waning; wanes.
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Definitions of waning from WordNet
a gradual decrease in magnitude or extent;
the waning of his enthusiasm was obvious
the waxing and waning of the moon
(of the Moon) pertaining to the period during which the visible surface of the moon decreases;