Shibboleth

Judges 12:6
(ear of corn, stream). Pronounced sibboleth by Ephraimites, and shibboleth by Gileadites. When the latter conquered the former, and held the fords of Jordan, they exacted the pronunciation of this word in order to distinguish friend from foe. Any other word beginning with sh would have answered the same purpose (Judg. 12:66Then said they unto him, Say now Shibboleth: and he said Sibboleth: for he could not frame to pronounce it right. Then they took him, and slew him at the passages of Jordan: and there fell at that time of the Ephraimites forty and two thousand. (Judges 12:6)).