In an extremely significant or shocking manner; with impact comparable to an earthquake.
Adverbial form of 'earthshaking,' using the '-ly' suffix (Old English -lice, indicating manner). A metaphorical elaboration combining geological intensity with descriptive power.
Adding '-ly' to 'earthshaking' creates a word that only makes sense metaphorically—there's no literal way for something to 'be shaking' in an earth-like manner, but the exaggeration is exactly the point.
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