I'm confused "deadly" with "deathly",could you help me?
In desperate housewives,
"First of all, your father can't eat onions. He's deathly allergic."

I searched it up in the dictionary,
deadly dangerous andable to kill:
Deathly seeming like death:
According to the definition，I think, it might be more appropriate to use "deadly" in this sentence.

So,could you tell me the differences between these two words and tell me why use "deathly" in the sentence?
Thanks a lot.

Question

I'm confused "deadly" with "deathly",could you help me?
In desperate housewives,
"First of all, your father can't eat onions. He's deathly allergic."

I searched it up in the dictionary,
deadly   dangerous andable to kill:
Deathly   seeming like death:
According to the definition，I think, it might be more appropriate to use "deadly" in this sentence.

So,could you tell me the differences between these  two words and tell me why use "deathly" in the sentence?
Thanks a lot.

Answer

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

"He's deathly allergic to onions."
The allergies are deathly. He could be mildly allergic, severely allergic, etc. It describes the intensity of the allergy.

"Due to his allergies, onions are deadly."
Onions are deadly because they would cause death, or cause the allergies that lead to death.