History / Where did the stereotype of Polish people being stupid come from? [131]

I am not 100% Polish, in fact I'm about every nationality. Every time I mention my great grandmother was Polish, I get the "dumb pollack" remark. Where exactly did it start? I've done so many internet searches and I can't come up with a definite answer. Some sites say it was the swift takeover by the Germans in WWII and others say it's because immigrants to America didn't speak much English. No immigrant speaks much English. Can anyone offer some insight on this stereotype? Thanks