'waningly' originates from English, specifically formed from the present participle 'waning' (from the verb 'wane') plus the adverbial suffix '-ly'.

'wane' changed from Old English 'wanian' (meaning 'to decrease') through Middle English forms such as 'wanen' and eventually became the modern English verb 'wane'; the forms 'waning' and 'waningly' were produced by adding the derivational suffixes '-ing' and '-ly'.

Initially it meant 'to decrease; become less', and over time the related forms broadened to mean 'fading or losing strength' and—influenced by the adjective 'wan'—occasionally 'pale or sickly' in modern usage.