A shibboleth is, more or less, a linguistic password used to identify a cultural group. For example, English visitors to Scotland or Wales can often be identified by their asking them to pronounce the place names “Loch Lomond”, in Scotland, or “Croesgoch”, in Wales. In both examples, they would be unlikely to pronounce the hard “ch” sound, as a native would.
The word shibboleth originates from a story of the Hebrew Bible. The Gileadites, having successfully occupied the land of Ephraim, were able to prevent the refugee Ephraimites from returning to the territory by asking them to “say Shibboleth”. The Ephraimites dialect lacked the sh sound; the Gileadites did not. According to the story, 42,000 Ephraimites were killed using this test.
This seems ridiculous, yet modern history has witnessed a similar atrocity. Over five days in October 1937, the Parsley Massacre saw up to 35,000 Haitians killed in the Dominican Republic. During the massacre, Dominican soldiers could identify Haitians by holding a sprig of parsley and asking them to speak its name. A Haitian would be unable to pronounce the trilled ‘r’ in the Spanish for parsley (perejil), and would be slaughtered.
Shibboleths aren’t all morbid, and don’t necessarily have to be based on sounds that can or can’t be made. Science fiction fans can identify one-another by their use of ‘sf’ rather than the mainstream ‘sci-fi’; natives of Toronto (and some other Canadians) by the dropping of the second ‘t’ when pronouncing their city name; and novice programmers sometimes by their use of ‘object orientated’ rather than ‘object oriented’. A long list of shibboleths can be found on wikipedia.
The U and non-U English debate of the 1950s is an interesting related piece of history, where the upper class (U English) and aspiring middle classes (non-U English) were supposedly identifiable by their use of language. Interestingly, the shibboleth words were often the reversal of what you might expect: the middle classes using “Dentures”, “Preserve” and “Pardon?”, but the upper class using “False Teeth”, “Jam” and “What?”. This was, of course, because the aspiring middle classes were trying to appear to be upper class, whereas the upper class were not so self-conscious.