An extract from the notes found in BHFA, Volume 5.
וַיֹּאמְרוּ לוֹ אֱמָר־ נָא שִׁבֹּלֶת וַיֹּאמֶר סִבֹּלֶת
sibbolet and he said shibbolet please say to him and they said
Noun, f. sg. abs. meaning, "flowing stream" (here and in Ps 69:3). The construct form is found in Ps 69:16, and in Is 27:12. Elsewhere it is found to mean, "ear of grain." In the abs. sg. in Jb 24:24. In the abs. pl. in Gen 41:5-27; Is 17:5,5; Ru 2:2. In the pl. cstr in Zec 4:12.
Noun, f. sg. abs. This is an example of a hapax legomenon (a word that is found only once in a body of work like the Hebrew Bible). It has no connection to any other root, making it impossible to ascertain with certainty what theoriginal author meant.
One only has to know the Hebrew alphabet to be able to see how the difference in pronunciation of the same word in different dialects was used by Jephthah as a code word to identify Ephraimites masquerading as Gileadites.