The quality or state of being deadly or capable of causing death.
From 'deadly' (capable of causing death) plus the noun-forming suffix '-ness' (state or quality). This combines Old English roots meaning 'death' with the abstract noun suffix to describe the abstract quality of being lethal.
The suffix '-ness' is one of English's most productive tools—you can stack it on almost any adjective to create an abstract noun, which is why 'deadliness' feels both grammatically perfect and somehow poetic when describing how dangerous something truly is.
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