M-W's background on the word:
The Bible's Book of Judges (12:4-6) tells the story of the Ephraimites, who, after they were routed by the Gileadite army, tried to retreat by sneaking across a ford of the Jordan River that was held by their enemy. The Gileadites, wary of the ploy, asked every soldier who tried to cross if he was an Ephraimite. When the soldier said "no," he was asked to say "shibboleth"(which means "stream" in Hebrew). Gileadites pronounced the word "shibboleth," but Ephramites said "sibboleth." Anyone who left out the initial "sh" was killed on the spot. When English speakers first borrowed "shibboleth," they used it to mean "test phrase," but it has acquired additional meanings since that time.
What M-W does not note is that the word "sibboleth" (the mispronounced version the Ephraimites used) means "ear" as in an ear of grain. This word "sibboleth" is only used once in the whole Bible... in the Judges 12 passage.
In the "Left Behind" series believers would often use a test phrase... a shibboleth. They would say "He is risen." and the response phrase was expected to be "He is risen, indeed." If someone did not respond appropriately they would know that person was not a believer.
Shibboleth and sibboleth (streams and ears) bring to mind another passage of Scripture in Revelation 22:
Then he showed me a river of the water of life, clear as crystal, coming from the throne of God and of the Lamb, in the middle of its street. On either side of the river was the tree of life, bearing twelve kinds of fruit, yielding its fruit every month; and the leaves of the tree were for the healing of the nations. (Revelation 22:1-2)
This passage begs the question: why do the nations need to be healed?
Won't everyone who is left be saved? Didn't we see those whose name is not written in the book of life throw into the fire in Revelation 20:15?
More to come on this as a study of end times is in the works...