I'm pretty sure it's the all-time-clown; you know, the one with the big smile and the red hair. The traditional one - but I'm not completely sure. The same as the one from "IT" - the movie.
and I'm also confident enough to say that I think it's also used as a thing to call others when they act stupid or ... like clowns. say you would be walking, and maybe whistling as well - and then you would see me and wave and turn a bit and accidently step on something that made you loose your balance and you then fall right in to a bucket of water with you behind landing first, - I would then, try and stop from laughing, stop waving and say; Hey, you bozo - you're suppose to drink with the other end.