In a manner that shows a wide opening or stare; with one's mouth open in surprise or wonder.
From 'gaping' (present participle of 'gap,' from Old Norse 'gapa' meaning 'to open the mouth') plus the adverbial suffix '-ly.' The sense evolved from physical opening to describe the manner of doing something with visible astonishment.
Medieval writers used 'gapingly' to describe how audiences watched miracles or spectacles—the word captures that specific moment when your brain is so surprised that your mouth literally falls open before you can control it.
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