"What am I...retarded?"

As I was leaving the gym today, I tried saying "what's up" to one of the hot trainers. But what came out was a bunch of gibberish. Not that he makes me nervous, it's just for some reason, I get tired of saying "Wie gehts dir?" (an informal "how are you?") so I tried something else - which didn't work. So, instead of being pleasant, I just looked (in the words of Jake Ryan's friend on Sixtenn Candles) retarded.

I mean, when I jokingly say hello to my friends back home, and others who speak English, I say funny greetings like, "What up dude?" and "Shello." No, I'm not being serious but it's nice to mix it up. I guess I shouldn't try to be funny in German. I just look foolish.