In a manner resembling harping; with a tedious, repetitive quality; in the manner of continuously complaining about something.
From 'harping' (the present participle of 'harp') combined with the adverbial suffix '-ly.' The word creates an adverb from the figurative verb meaning to talk repetitively.
This adverb is practically obsolete, but it perfectly captures something we all experience—the tone of voice someone uses when they keep bringing up the same complaint, like a broken record or, aptly, a harp being played endlessly.
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