It's a word from the Hebrew Bible that was never given a meaning (from II Samuel, I think).
There was a story of a conflict between the tribes of Israel, and the Jews on the west side of the Jordan were afraid of infiltrators. So they set up a bridge over the river and tested people as they came across. If they could say the word "shibboleth" with the proper accent, they were allowed across. If they said it with the accent of the eastern tribes, they were barred from crossing.
