In modern English, a shibboleth is a practice or social custom that makes obvious one's origins. In Biblical Times, assuming the Bible is a valid historical source, it was the word that caused the death of thousands.
Shibboleth is from the hebrew word "shibbólet," (שִׁבֹּלֶת), meaning either 'a portion of plant containing grains' or 'quickly running stream.' It was used by the Ephraimites, whose dialect lacks a postalveolar fricative, to pick out and slaughter the Gileadites, whose dialect did not lack such a sound.
5And the Gileadites took the passages of Jordan before the Ephraimites: and it was so, that when those Ephraimites which were escaped said, Let me go over; that the men of Gilead said unto him, Art thou an Ephraimite? If he said, Nay; 6 Then 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 12:5-6 KJB
Using shibboleths is a common method to identify infiltrators in the armed forces, especially during times of increased paranoia during war.
- Knowledge of baseball was used by Americans to find German infiltrators during World War II.
- The Dutch used the word 'Scheveningen,' a part of The Hague to find Germans. In German, it would be pronounced like an English sh, but in Dutch it contains a velar fricative.
- Also during World War II, Nazi soldiers would force boys to show them their genitals. If they were circumcised, a Jewish practice, they would be removed and relocated to one of the terrible concentration camps.
- Yet again during World War II, people in American uniforms were told to pronounced 'lollapalooza.' If the person was a Japanese spy, it would be pronounced 'rorr-' and he would be shot on the spot.
When speaking through a telephone or other method where the person being addressed is unseen, dialectal differences can be shibboleths. Dialectal differences, such as the droping of final r's, affricativiaztion of /t/, especially before the word 'you,' and usage of slang are all rather obvious indicators of origin and possibly race.