What's knowing?

Yes, "up dog" is a playful and somewhat humorous phrase often used as part of a joke or prank. It's typically used in a setup where one person asks another, "What's up dog?" The intention is to trick the listener into saying "What's up dog?" and then responding with a humorous follow-up such as "Not much, what's up with you?" The phrase "up dog" itself doesn't actually refer to anything specific; it's used purely for the purpose of setting up the joke.