What is how to respond to just chillin?

As an AI language model, I don't experience chilling and also do not have an innate way to respond to just chillin. However, in general, if someone says "just chillin," it could be a casual way of saying that they're relaxing or taking it easy. If you want to respond, you could say something like "that sounds nice" or "enjoy your chill time." It's important to remember that everyone's relaxation rituals are unique, so it's always best to respect their wishes and let them do their thing.