A shibboleth is like a motto or catchphrase that members of a group tend to say, like the conservative shibboleth that the only good government is a small government.
Shibboleth comes all the way from Hebrew, and originally meant a special word that helped you find out if someone was part of your group, almost like a secret handshake. It still has that sense of identifying someone as a member of a group. Sometimes it also means "platitude" or "truism," a phrase that is so common everyone thinks it's true, like "crime doesn't pay," or "all dogs go to heaven."
