deadly (deadlier comparative) (deadliest superlative )
1 adj If something is deadly, it is likely or able to cause someone's death, or has already caused someone's death.
(=lethal, fatal)
He was acquitted on charges of assault with a deadly weapon.
2 adj If you describe a person or their behaviour as deadly, you mean that they will do or say anything to get what they want, without caring about other people., (disapproval)
The Duchess levelled a deadly look at Nikko.
3 adv You can use deadly to emphasize that something has a particular quality, especially an unpleasant or undesirable quality., (emphasis)
(=deathly)
Broadcast news was accurate and reliable but deadly dull...
4 adj A deadly situation has unpleasant or dangerous consequences.
...the deadly combination of low expectations and low achievement...
5 adj Deadly enemies or rivals fight or compete with each other in a very aggressive way.
The two became deadly enemies...