E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
The password of a secret society; the secret by which those of a party know each other. The
Ephraimites quarrelled with Jephthah, and Jephthah gathered together the men of Gilead and fought with Ephraim. There were many fugitives, and when they tried to pass the Jordan the guard told them to say Shibboleth, which the Ephraimites pronounced Sibboleth, and by this test it was ascertained whether the person wishing to cross the river was a friend or foe. (Judges xii. 116.)