a Fabian

There actually are different meanings of "being a Fabian".
Either, they act idiotic and reckless at other people's expense without knowing they're annoying.
Or they act like dummies and do something silly. People usually don't get when they are a Fabian - but in fact, everyone else does.

In either way, if you called "a Fabian", you should think about your actions and what effect they do have to other people.

(1)
Person 1: *"accidentally" nudges someone, laughs*
Person 2 (a friend): "Dude, you're such a Fabian..."

(2)
Person 1: *walking down a street; talkes to Person 2*
*crashes into a road sign* "Ouch.."
Person 2: *shakes head* "Wow, you're such a Fabian !"