Aiming or willing to destroy; implacable; desperately hostile; flagitious; as, deadly enemies
emphasis You can use deadly to emphasize that something has a particular quality, especially an unpleasant or undesirable quality. Broadcast news was accurate and reliable but deadly dull = deathly
Capable of causing death; mortal; fatal; destructive; certain or likely to cause death; as, a deadly blow or wound
(of a disease) having a rapid course and violent effect causing or capable of causing death; "a fatal accident"; "a deadly enemy"; "mortal combat"; "a mortal illness"
If something is deadly, it is likely or able to cause someone's death, or has already caused someone's death. He was acquitted on charges of assault with a deadly weapon. a deadly disease currently affecting dolphins Passive smoking can be deadly too = lethal, fatal