shibboleth

N-COUNT

If you describe an idea or belief as a shibboleth, you mean that it is thought important by a group of people but may be old-fashioned or wrong.[ FORMAL ]

• It is time to go beyond the shibboleth that conventional forces cannot deter.
  

.... Cobuild

Modern Usage of  shibboleth: 

In numerous cases of conflict between groups speaking different languages or dialects, one side used shibboleths in a way similar to the above-mentioned Biblical use, i.e., to discover hiding members of the opposing group. Christians might have been familiar with the Biblical story and directly inspired by it, or might have independently invented the same method under similar circumstances. Modern researchers use the term "Shibboleth" for all such usages, whether or not the people involved were using it themselves.

Today, in the English language, a shibboleth also has a wider meaning, 

• referring to any "in-crowd" word or phrase that can be used to distinguish members of a group from outsiders - even when not used by a hostile other group. 
• The word is also sometimes used in a broader sense to mean jargon, the proper use of which identifies speakers as members of a particular group or subculture.
• Shibboleths can also be customs or practices, such as male circumcision, or a signifier, such as a semiotic.
• Cultural touchstones and shared experience can also be shibboleths of a sort. For example, people about the same age tend to have the same memories of popular songs, television shows, and events from their formative years. Much the same is true of alumni of a particular school, veterans of military service, and other groups. Discussing such memories is a common way of bonding. In-jokes can be a similar type of shared-experience shibboleth.

....Wikipedia

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1. A word or pronunciation that distinguishes people of one group or class from those of another.

2(a). A word or phrase identified with a particular group or cause; a catchword.

2(b). A commonplace saying or idea.

3. A custom or practice that betrays one as an outsider.