Shibboleth

shibboleth \ˈshi-bə-ləth\ noun: 1 a. a word or saying used by adherents of a party, sect or belief and usually regarded by others as empty of real meaning {the old shibboleths come rolling off their lips — Joseph Epstein}, b. a widely held belief {today this book publishing shibboleth is a myth — L. A. Wood}, c. truism, platitude {some truth in the shibboleth that crime does not pay — Lee Rogow},
2 a. a use of language regarded as distinctive of a particular group {accent was a shibboleth of social class — Vivian Ducat}, b. a custom or usage regarded as distinguishing one group from others {for most of the well-to-do in the town, dinner was a shibboleth, its hour dividing mankind — Osbert Sitwell}

Etymology: Hebrew shibbōleth stream, part of a plant containing grains (from the use of this word in Judges 12:5-6 as a test to distinguish Gileadites from Ephraimites)

Wikipedia fleshes out the etymology:

In an account from the Hebrew Bible the pronunciation of the word shibboleth was used to distinguish Ephraimites, whose dialect lacked a sh sound (as the sh in shoe), from Gileadites, whose dialect did include such a sound.

After the inhabitants of Gilead inflicted a military defeat upon the tribe of Ephraim (circa 1370–1070 BC), the surviving Ephraimites tried to cross the Jordan River back into their home territory, but the Gileadites secured the river’s fords to stop them. In order to identify and kill these refugees, the Gileadites put each refugee to a simple test:

“Gilead then cut Ephraim off from the fords of the Jordan, and whenever Ephraimite fugitives said, ‘Let me cross,’ the men of Gilead would ask, ‘Are you an Ephraimite?’ If he said, ‘No,’ they then said, ‘Very well, say shibboleth.’ If anyone said, sibboleth, because he could not pronounce it, then they would seize him and kill him by the fords of the Jordan. Forty-two thousand Ephraimites fell on this occasion.” — Judges 12:5-6