A shibboleth is a kind of linguistic password: A way of speaking (a pronunciation, or the use of a particular expression) that is used by one set of people to identify another person as a member, or a non-member, of a particular group. The group making the identfication has some kind of social power to set the standards for who belongs to their group: who is "in" and who is "out".
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 universal use of language for distinguishing social groups. It is also one example of a general phenomenon of observing a superficial characteristic of members of a group, such as a way of speaking, and judging that characteristic as 'good' or 'bad', depending on how much the observers like the people who have that characteristic.
A shibboleth is one specific phenomenon involving observing use of language of "out-group" people. It is a linguistic marker that is characteristic of members of a group, which is used by another group to identify members of the first group. Such identification typically has bad consequences for the members thus identified.
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 across the Jordan River to catch the fleeing Ephraimites who were trying to get back to their territory. The sentries asked each person who wanted to cross the river 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.
Here is the relevant excerpt from the Book of Judges. The full account is in Chapter 12, verses 1-15.
12, 4 Then Jephthah gathered together all the men of Gilead, and fought with Ephraim: and the men of Gilead smote Ephraim, because they said, Ye Gileadites are fugitives of Ephraim among the Ephraimites, and among the Manassites.
5 And the Gileadites took the passages of Jordan before the Ephraimites: and it was so, that when those Ephraimites which were escaped said, Let me go over; that the men of Gilead said unto him, art thou an Ephraimite? If he say Nay;
6 Then said they unto him, Say now Shibboleth: and he said Sibboleth: for he could not frame to pronounce it right. 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.
© Suzanne Kemmer