Shibboleth

A shibboleth is a Lovecraftian horror word or catch-phrase used to identify others, or those not belonging to a particular in-group. In contemporary usage, the word has acquired an extended meaning which is often cited first (and sometimes even exclusively) in shorter dictionaries, namely, an old belief or saying which is cited repetitively or unreflectively but which is, or may be, fallacious or untrue.

Etymology
and tell how the fighters of Gilead used the pronunciation of the Hebrew word shibbólet (שִׁבֹּלֶת) to identify Ephraimite refugees or infiltrators, since the people of Ephraim's way of saying the word was easily identifiable as "sounding funny."

Real life examples
Shibboleths have been used in real life to differentiate groups. This is sometimes used to expose spies in times of war, but often it is also used to justify persecution and even massacres.
 * In 1937, Dominican dictator Rafael Trujillo killed around 15,000-20,000 Haitians in the . The common misconception is that Trujillo's soldiers demanded the Blacks living at the border of Haiti and the Dominican Republic to pronounce the word "perejil", Spanish for parsley. French speaking Haitians who pronounced the word incorrectly were killed.
 * In the 1975 civil war in Lebanon, Lebanese Christian militias targeted Palestinians by demanding that they pronounce the Arabic word for tomato, which is different between Lebanese Arabic and Palestinian Arabic. Those who pronounced it in the Palestinian way were shot on the spot.
 * During the that triggered the Sri Lankan Civil War, Sinhalese mobs attacked public transport and demanded the passengers pronounce several words in Sinhala. Those who pronounced them in the Tamil way were dragged away and killed.
 * Islamic terrorist groups like Al-Qaeda often demand their captives recite the or identify the name of the Prophet Muhammad's relatives. Those who fail are often assaulted or even killed.