shibboleth

Then the Gileadites took the fords of the Jordan against the Ephraimites. Whenever one of the fugitives of Ephraim said, “Let me go over,” the men of Gilead would say to him, “Are you an Ephraimite?” When he said, “No,” they said to him, “Then say Shibboleth,” and he said, “Sibboleth,” for he could not pronounce it right. Then they seized him and killed him at the fords of the Jordan. Forty-two thousand of the Ephraimites fell at that time.

Judges 12:5-6