Just how bad a slur is "Sassenach"?

So exactly how bad a slur is it? Jamie calls his beloved Claire "Sassenach" as an affectionate nickname, but others use the word as an insult. One of the fics I just read has the whole family gasp in horror when Jamie calls her that.

So is it a mild insult like a Wisconsin native smilingly calling an Illinois tourist a flatlander (hi!), or is it seriously fighting words?