The New Student's Reference Work/Shibboleth

Shibboleth (shĭb′ō̇-lĕth), a word used by the Gileadites under Jephtha after their victory over the Ephraimites, to test the fugitives at the ford. The Ephraimites could not pronounce the sh, but called it sibboleth, and so were easily known. The word is still used in the sense of a test.