[Laughs, too, shaking his head -- air, indeed... and then he just laughs harder. Arthur, you loon. Eventually he manages to stop laughing, bumping his shoulder against Merlin's.]
You're my friend, remember? I'm pretty sure that implies I don't exactly hate you. But I guess nobles have never been able to boast being particularly bright. [He knows it applies to himself, too, okay, it's a double joke.]