In a manner that is confusing, overwhelming, or astonishing; in a way that boggles the mind.
From 'boggling' (boggle + '-ing') + '-ly' (Old English adverbial suffix). The '-ly' suffix transforms adjectives into adverbs describing how an action occurs.
English speakers invented an elaborate suffix chain—boggle → boggling → bogglingly—where each suffix adds a layer of grammatical transformation, showing that our language constantly creates new words by stacking these tiny grammatical units.
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