ah, now you're asking questions! baby steps. it's a word invented by 1800s German antisemites to sound more politically correct than "Judenhass" (Jew hate). they wanted to cast bigotry as opposing foreign "Semitic" influence on good old Germanic culture. 1800s racists doing 1800s racism!
this is why "antisemitism" refers to hatred specifically of Jews, and there are other words for hatred of Arabs and other peoples formerly called "Semitic". similarly, "colored" in the United States has never referred to people whose color was white. words are not just their etymologies!