shibboleth

shibboleth \SHIB-uh-lith; -leth\, noun:

1. A peculiarity of pronunciation, behavior, mode of dress, etc., that distinguishes a particular group of persons.
2. A slogan; a catchword.
3. A common saying or belief with little current meaning or truth.

Accustomed to the veneer of noise, to the shibboleths of promotion, public relations, and market research, society is suspicious of those who value silence.
-- John Lahr

The fish oil shibboleth is only the latest to be overturned in recent years. Vitamin supplements and fibre have also been found to provide no benefits.
-- Nigel Hawkes, "Nice idea, but where's the proof?", Times (London), March 24, 2009

Class size is another shibboleth: First, small class sizes do not increase learning, and, second, class sizes have become quite small anyway.
-- Jay Nordlinger, "The Anti-Excusers", National Review, October 27, 2003

Shibboleth is from Hebrew shibboleth, "stream, flood," from the use of this word in the Bible (Judges 12:4-6) as a test to distinguish Gileadites from Ephraimites, who could not say 'sh' but only 's' as in 'sibboleth'.