In a Bible story, two Israelite groups fight. The losing tribe, Ephraim, must cross the Jordan River to get home.
The winners guard the river crossings. Any man who says he isn't from Ephraim must say "shibboleth." Men from Ephraim say "sibboleth," so they're caught and killed.
So in English, a shibboleth is a word or habit that shows someone's group.