In a deathful manner; in a way that is full of death or fatality; deadly.
From deathful + -ly suffix (forming adverbs from adjectives). The -ly suffix comes from Old English -lice, originally meaning 'in the manner of.'
Archaic adverbs ending in -fully (like 'deathfully' and 'painfully') show how English created descriptive language by stacking suffixes—each one adding a layer of modification.
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