Dude is an informal word for a person, especially a guy or man. Originally, in the late 1800s, it meant a fancy city fellow who dressed stylishly but didn't know much about practical things like ranch work or outdoor skills. Cowboys used it somewhat mockingly for tourists visiting the West, which is why vacation ranches are still called dude ranches.

Over time, the meaning shifted completely. By the 1960s and 70s, especially among surfers in California, dude became a casual, friendly way to address someone, like “Hey, dude, want to come over?” Today, people use it for almost anyone, regardless of gender, though it still leans toward guys.

The word also works as an exclamation showing surprise, excitement, or emphasis: “Dude! That was an amazing catch!” or “Dude, I can't believe we have a pop quiz today.” In this usage, it's like saying “Wow!” or “No way!”

Dude is conversational and relaxed. You'd say it with friends at recess, not in a formal letter to your principal. While it's become so common that most people barely think about it, it's still casual language, so it works better in some situations than others.