I have to supply some context to Webster´s definition of Fuchs. While technically correct, it is a rather oldfashioned expression and nobody actually calls a first year student a Fuchs. Except the guys in student associations, called Burschenschaft in German. And, you see, most of these Burschenschaften are very conservative and right-wing, therefore they have a dubious reputation. Being a Fuchs in a Burschenschaft means that you are new and you will be bullied. The bright side is that you will be the one to bully the new Füchse next year, if that sort of thing is attractive for you.

So keep in mind, nobody will understand if you call him a Fuchs except if he´s in a Burschenschaft.

Come to think of it, people would understand "Du bist ein Fuchs!", but in a completely different way - you might say it when somebody has just done something very clever.