The term "deadly duo" is a figurative expression that refers to two individuals or things that are particularly dangerous, destructive, or harmful when combined. It can be used in various contexts such as describing a pair of criminals who commit violent crimes together, a combination of diseases that have high mortality rates if contracted simultaneously, or even a powerful duo of athletes on the same sports team. The phrase emphasizes the idea that these two elements are particularly dangerous when they work together and can cause significant harm or damage.