shibboleth

A unique pronunciation, word, behavior, or practice used to distinguish one group of people from another, and to identify individuals as either members of the group or outsiders.

A shibboleth is a kind of linguistic password: A way of speaking (a pronunciation, or the use of a particular expression) that identifies one as a member of an 'in' group. The purpose of a shibboleth is exclusionary as much as inclusionary: A person whose way of speaking violates a shibboleth is identified as an outsider and thereby excluded by the group. (This phenomenon is part of the "Judge a book by its cover" tendency apparently embedded in human cognition, and the use of language to distinguish social groups).

The story behind the word is recorded in the biblical Book of Judges. The word shibboleth in ancient Hebrew dialects meant 'ear of grain' (or, some say, 'stream'). Some groups pronounced it with a sh sound, but speakers of related dialects pronounced it with an s.

In the story, two Semitic tribes, the Ephraimites and the Gileadites, have a great battle. The Gileadites defeat the Ephraimites, and set up a blockade to catch the fleeing Ephraimites. The sentries asked each person to say the word shibboleth. The Ephraimites, who had no sh sound in their language, pronounced the word with an s and were thereby unmasked as the enemy and slaughtered.