adjective deadlier -·lier, deadliest -·li·est

1. causing death or likely to cause death a deadly poison
2. to the death; mortal or implacable deadly combat, deadly enemies
3. typical of death deadly pallor
4. very harmful; destructive
5. 1. extreme or excessive deadly silence
   2. out-and-out; utter with deadly gravity
6. oppressively tiresome a deadly bore
7. perfectly accurate deadly aim
8. Theol. causing spiritual death the seven deadly sins

Etymology: ME dedlich < OE deadlic: see dead & -ly

adverb deadlier -·lier, deadliest -·li·est

1. in a way suggestive of death to lie deadly still
2. extremely or excessively deadly serious