How did a sound become a life-or-death password?

In the biblical Book of Judges, after defeating Ephraim, the Gileadites held the fords of the Jordan. Anyone who asked to cross and denied being an Ephraimite was told to say “shibboleth.” An Ephraimite couldn’t make the sound and said “sibboleth,” and was killed at the river. The text says 42,000 Ephraimites died at that time.

The word itself was never secret. The accent was the proof, which makes it closer to a voice test than a password: it checked who you were, not what you knew.

English kept the word for that kind of test. A shibboleth is now a word, phrase, or custom known only to a particular group, one you can use to prove you really belong.