In a way that is very serious, severe, or causing great pain or harm.
From Old French 'grievous' (heavy, serious, severe) via Latin 'gravis' (heavy, weighty). The suffix '-ly' converts the adjective to an adverb. The sense evolved from physical heaviness to metaphorical weightiness in terms of seriousness or severity.
Medieval writers loved the word 'grievously'—it appears constantly in historical records to describe serious crimes or wounds, which shows how the word became essential for expressing that something wasn't just bad, but genuinely catastrophic.
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