Shibboleth - a use of language regarded as distinctive of a particular group
A person's pronunciation of words provides many clues for determining where he lives or comes from.
In the twelfth chapter of the Book of Judges, there is an account of a battle between the Gileadites and the Ephraimites in which a test of pronunciation was used to distinguish members of the opposing armies. The Ephraimite army was routed, and in their retreat they attempted to cross the Jordan river at a ford held by the Gileadites. Anyone wishing to pass was asked by the Gileadites if he were an Ephraimite. If the reply was "No" he was then asked to say the word shibboleth. In Hebrew shibboleth may mean either 'an ear of grain' or 'a stream', but on that occasion its meaning was of no importance. Unlike the Gileadites, the Ephraimites were unable to pronounce an sh sound. Thus if the reply were "sibboleth" the Gileadites knew the speaker was an Ephraimite; "then they took him and slew him at the passages of Jordan: and there fell at that time of the Ephraimites forty and two thousand" (Judges 12:6, Authorized Version).
In English, shibboleth was borrowed from this passage and has come to mean 'a use of language or custom regarded as distinctive of the members of a particular group'. From this it has also developed the sense of 'a slogan or catchword used by a particular group'.