Yes and I believe the term derives from "Bohemia" which was a region in Greater Moravia prior to World War 1. In the late 1800's many people from the region immigrated to the United States which spawned the 'racial' hatred that in turn spawned the term: Bohunk.

That offended you? I think this word is overused.

Not surprising.
To me the most offensive thing was Sam getting all pissy because the girl with a head brace sat next to her. OK, I get it. Being a sophomore in high school sucks, sometimes. She's insecure, awkward, at a high school with a dog-eat-dog social structure. BUT who is she to look down on this girl just because she has freaking orthodontic issues? She's not exactly a "winner", right? It's realistic, but that just crossed a line somehow. In retrospect anyway.

What behavior is 'offensive'?

It's a comedy.
Schrodinger's cat walks into a bar and doesn't.

Sixteen Candles is full of offensive behavior and stereotypes. It's quite clear the movie is a product of its time and Hughes' white, privileged upbringing.
And I say all of that as a fan of the movie. Times and social norms change.

I never understood that, I knew that it wasn't nice but that's pretty nasty.

"Bohunk" is a disparaging slur against people (or the descendants of people) who immigrated from Central Europe, especially Hungary. In addition to being an ethnic slur, it has class connotations. Basically, it means that the Bakers see the sister's fianc and eventual husband as being from an immigrant-descended family that is lower-class and less cultured than the Bakers.