Shibboleth – N., from Hebrew: any distinguishing practice which is indicative of one’s social or regional origin. It usually refers to features of language, and particularly to a word whose pronunciation identifies its speaker as being a member or not a member of a particular group.
(following from wikipedia):
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.