In a way that causes death, or extremely; to a severe degree that's nearly unbearable.
From Latin 'mortalis' (subject to death), from 'mors' (death). The '-ly' adverbial suffix converts the adjective into an adverb describing manner or degree.
Shakespeare loved this word—he'd write 'mortally wounded' or 'mortally ashamed,' using 'mortally' to intensify emotions as if feelings could literally kill, showing how death metaphors saturate emotional language!
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