Deadly is an adjective used to describe something that is capable of causing death, while deathly is an adjective used to describe something that is related to death or has a deathly appearance.

Deadly may imply no more than an extreme of something or it may suggest a disgusting extreme of some depressing or spirit-destroying quality. Deathly applies only to that which suggests the appearance or the presence of death.

The adjective deadly means lethal or likely to cause death. The adjective deathly means suggestive of death.

Deadly has several meanings, among them "fatal" (a deadly disease),"relentless" (a deadly enemy),"like death" (a deadly paleness), and "boring" (a deadly speech). Deathly means "resembling death" and "utterly": "There was a deathly odor from the tomb." "I was deathly afraid of the huge guns."

Deathly means having the quality of death. Deadly means having the ability to kill or cause death.