shibboleth
A particular pronunciation or custom that is regarded as distinguishing members of a group from non-members, especially when used as a test.
In recent times, the correct treatment of initial h in speech has come to be regarded as a kind of shibboleth of social position.
A common or longstanding belief, custom, or catchphrase associated with a particular group, especially one with little current meaning or truth.
It's about time we abandoned the bourgeois shibboleth that earning money makes you a better person.
Where it comes from
Learned borrowing from Biblical Hebrew שִׁבֹּלֶת /שיבולת (šibbṓleṯ, “ear [of wheat] or stream, torrent”). Doublet of sebboleth and Middle English sebolech, both from Late Latin sebboleth in the Vulgate.
Adapted from the Wiktionary entry shibboleth (CC BY-SA 4.0).