A shibboleth is a word or a phrase that distinguishes one group of people from another.
Wikipedia has many examples (some rather funny):
- Some United States soldiers in the Pacific theater in World War II used the word lollapalooza as a shibboleth to challenge unidentified persons, on the premise that Japanese people often pronounce the letter L as R or confuse Rs with Ls. A shibboleth such as "lollapalooza" would be used by the sentry, who, if the first two syllables come back as rorra, would "open fire without waiting to hear the remainder".
- During World War II, a homosexual US sailor might call himself a "friend of Dorothy", a tongue-in-cheek acknowledgment of a stereotypical affinity for Judy Garland in The Wizard of Oz. This code was so effective that the Naval Investigative Service, upon learning that the phrase was a way for gay sailors to identify each other, undertook a search for this "Dorothy", whom they believed to be an actual woman with connections to homosexual servicemen in the Chicago area.
- In Cologne, a common shibboleth to tell someone who was born in Cologne from someone who had moved there is to ask the suspected individual, Saag ens "Blodwoosch" (say "blood sausage", in Kölsch). However, the demand is a trick; no matter how well one says Blodwoosch, they'll fail the test; the correct answer is to say a different word entirely; namely, Flönz, the other Kölsch word for blood sausage.
In many professions, one may have shibboleths as well, to distinguish those who are more experienced or knowledgeable from those who are not. These may not be used in a password-like manner, rather in a manner like this: "If they use certain words or concepts correctly, it's safe to assume they aren't new to this field".