|Easton's Bible Dictionary|
River, or an ear of corn. The tribes living on the east of Jordan, separated from their brethren on the west by the deep ravines and the rapid river, gradually came to adopt peculiar customs, and from mixing largely with the Moabites, Ishmaelites, and Ammonites to pronounce certain letters in such a manner as to distinguish them from the other tribes. Thus when the Ephraimites from the west invaded Gilead, and were defeated by the Gileadites under the leadership of Jephthah, and tried to escape by the "passages of the Jordan," the Gileadites seized the fords and would allow none to pass who could not pronounce "shibboleth" with a strong aspirate. This the fugitives were unable to do. They said "sibboleth," as the word was pronounced by the tribes on the west, and thus they were detected (Judges 12:1-6). Forty-two thousand were thus detected, and
"Without reprieve, adjudged to death, For want of well-pronouncing shibboleth."
Noah Webster's Dictionary
1. (n.) A word which was made the criterion by which to distinguish the Ephraimites from the Gileadites. The Ephraimites, not being able to pronounce sh, called the word sibboleth. See Judges xii.
2. (n.) Also in an extended sense.
3. (n.) Hence, the criterion, test, or watchword of a party; a party cry or pet phrase.
Int. Standard Bible Encyclopedia
shib'-o-leth (shibboleth): A test of speech applied by the men of Gilead to the Ephraimites, who wished to cross the Jordan, after defeat. If they pronounced the word cibboleth, their dialectic variety of speech betrayed them. (Judges 12:6). The word probably has the sense of stream or "flood" (compare Psalm 69:2).
Shibboleth (1 Occurrence)
Judges 12:6 then said they to him, "Now say'Shibboleth;'" and he said "Sibboleth;" for he couldn't manage to pronounce it right: then they laid hold of him, and killed him at the fords of the Jordan. At that time, forty-two thousand of Ephraim fell. (WEB KJV JPS ASV BBE DBY WBS YLT NAS RSV NIV)