Seeing as how nobody ever uses the term “Semites” to describe Jews anymore, and how “Semitic” refers to more peoples than just the Jews in the first place, why isn’t the term simply “anti-Jewish?”
If someone is against the Irish, they’re not “anti-Celtic,” and if they’re against the Spanish they’re not “anti-Iberian.” So why is it that only the term for an anti-Jewish bias uses a word from the ancient world?
I believe that the “anti-Jewish”/“anti-Judaism” construction is used to indicate a bias against the religion but not the “race.”
Since “Jew” is used to refer both to an ethnicity and a religion, and since sometimes there is a need to specify one use over the other (not all ethnic Jews practice the Jewish religion. Not everyone who practices the religion is ethnically Jewish), I think that “anti-semitic” arose as a way to distinguish a racially/ethnically motivated bias from a purely religious one.
Yeah, but it’s still a very clumsy solution, since, (as near as I can make out,) ‘Semitic’ is not another word for the Jewish ethnicity, but for a bundle of different ethnicities who arose in that area, some of whom are the most likely to be accused of anti-Jewish feelings (either for the race or the ethnicity or both.)
Thus, you have the curious situation of Semitic people being accused of being anti-Semitic, and not in an ironic “They hate themselves” way.
(In case it isn’t clear, yes I am thinking of ethnic arabs - I know they don’t all hate Jews, but it’s hard to escape the notion that some do.)
Interesting. According to this wiki article, the term probably first arose among anti-semitic German pamphleteers in the 19th century who applied it to their own ideology as an attempt to make it sound more scientific. In other words, anti-semites created the construction themselves. I guess any imprecision in the root words can be squarely blamed on them.
Yeah, that was a key moment in Jewish history. Up until then, people stopped hating us as soon as we converted to Christianity. As soon as psudeoscience got thrown into the mix things went from bad to worse.
Just like the Germans to be so imprecise! 
“Anti-Semite” suffers from the same problem as probably most titles used to describe collective groups of people, along the lines of “African American” or “Anglo-American,” basically being overly vague or even misleading. They stay in use cause everyone accepts them in their given meanings (ie: Anglo-Americans are white people, even if the white people may be of Slavic, Germanic, or even Asian ancestry depending on where their ancestors came from)