Real Dictionary

deadly
adjective: If something is deadly, it is likely or able to cause someone's death, or has already caused someone's death.
adjective: 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.
adjective: If you describe someone or something as deadly, you mean that you think they are very dull and boring.

deadly in American English
causing death or likely to cause death
adjective: causing or tending to cause death; fatal; lethal
adverb: in a manner resembling or suggesting death

deadly in British English
like death in appearance or certainty
adjective: likely to cause death