If something is deadly, it is likely or able to cause someone's death, or has already caused someone's death.

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.

If you describe someone or something as deadly, you mean that you think they are very dull and boring.

You can use deadly to emphasize that something has a particular quality, especially an unpleasant or undesirable quality.

A deadly situation has unpleasant or dangerous consequences.

Deadly enemies or rivals fight or compete with each other in a very aggressive way.

In sport, deadly players and actions are extremely skilful and successful.