The word shibbolethÂ (/shib-?-leth/ has come to mean any word, phrase, pronunciation, or mannerism that one group uses to classify members of another group as outsiders or enemies. One speaks of social shibboleths, political shibboleths, and religious shibboleths.
The most recent definition of shibboleth in the Oxford English Dictionary (OED) is
a moral formula held tenaciously and unreflectingly, especially a prohibitive one; a taboo.
In ancient Hebrew, shibboleth was just an ordinary word meaning “flood, stream,” or “ear of corn.” It acquired the sense of a test word for telling friend from enemy from its use by Jephthah the Gileadite against his enemies the Ephraimites.
The Gileadites took possession of fords along the Jordan river where they intercepted the fleeing Ephraimites. Since there was nothing about the appearance of the Ephraimites to distinguish them from the Gileadites, some other way of identifying them was needed. Both groups spoke Hebrew, but with different dialects. The Gileadites pronounced shibboleth with a “sh” sound, but the Ephraimites pronounced the sh as “s.” Jephthah’s men asked anyone who wanted to cross the Jordan to say “shibboleth.” If the man couldn’t pronounce the “sh” sound,
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, KJV
Various dialects serve as shibboleths between different segments of the US population, a good reason for learning to speak and write a form of Standard English.