More deadly or dangerous; comparative form of deadly, meaning more likely to cause death or more harmful.
From 'deadly' (capable of causing death, from Old English 'deaþ' + '-ly' meaning 'characteristic of'), with the comparative suffix '-er' added to compare two things.
Deadly is one of the oldest intensifying adjectives in English—it originally meant 'pertaining to death' and then got flipped to mean 'extremely' or 'very' (like 'deadly dull'), so 'deadlier' can mean both 'more fatal' and 'more extremely'.
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