In a feeblehearted manner; with lack of courage or determination; in a timid or cowardly way.
From 'feeblehearted' + '-ly' (in the manner of). Developed as an adverbial form in early modern English.
Reading old literature, you notice characters doing things 'feebleheartedly'—it was a very specific way to judge behavior, linking courage directly to moral character.
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