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sufficient to cause death:

«…にとって» 死を引き起こす, 死に至る, 致命的な «to» .

e.g. a lethal cocktail of alcohol and pills.

harmful or destructive:

死ぬほど強烈な, 恐ろしい.

e.g. the Krakatoa eruption was the most lethal on record.

late 16th century (in the sense ‘causing spiritual death’): from Latin lethalis, from lethum, a variant (influenced by Greek lēthē ‘forgetfulness’), of letum ‘death’.