In an exhausting, extremely demanding, or severely taxing manner; with grueling intensity.
From the adjective 'grueling' (from 'gruel' used as a verb) plus the adverbial suffix '-ly', converting the quality into a description of how something is done.
Adverbs like 'gruelingly' show English's ability to intensify emotional language—by adding '-ly' we can describe not just hard work, but the *feeling* of enduring it.
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