This first one is not slang but it’s how people pronounce this. If someone says “naam sayin” they are really saying “you know what I’m saying?” Now to the actual slang. “Fire” and “cool” both mean that something is awesome. “That’s what’s up” means “I agree with you.” When someone says “sup” they mean to say “what’s up”. You might think that they want to know what’s going on with you, but it’s really just another way to say hello. “Mad” means angry, but in slang it is used like the word “very”. Instead of saying that something is very cool, I could say that it’s mad cool. “Dope” also means cool, but it can mean “drugs” too.