Easton's Bible Dictionary (1897)
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 (Judg. 12:1-6).
Smith's Bible Dictionary (1863)
(a stream), (Judges 12:6) is the Hebrew word which the Gileadites under Jephthah made use of at the passage of the Jordan, after a victory over the Ephraimites, to test the pronunciation of the sound sh by those who wished to cross over the river. The Ephraimites, it would appear, in their dialect substituted for sh the simple sound s ; and the Gileadites, regarding every one who failed to pronounce sh as an Ephraimite and therefore an enemy, put him to death accordingly. In this way there fell 42,000 Ephraimites. There is no mystery in this particular word.
Any word beginning with the sound sh would have answered equally well as a test.
Hastings' Dictionary of the Bible (1898) & Schaff's Bible Dictionary
SHIB'BOLETH (an ear of corn, or a stream, a flood). Jud 12:6. In the course of a war between the Ephraimites and the Gileadites, the former were routed and fled toward the Jordan. The Gileadites had taken care to post a party at the fords, and when an Ephraimite who had escaped came to the river-side and desired to pass over, they asked him if he were not an Ephraimite. If he said, "No," they bade him pronounce "Shibboleth," and if he pronounced it "sibboleth," according to the dialect of the Ephraimites, they killed him. Thus fell 42,000 Ephraimites in a single day. Comp. Matt 26:73.
Milton says, with reference to that event, The word is now used for a test or the watchword of a party.
Hitchcock's Bible Names (1869)
Sibboleth, ear of corn; stream or flood