פּירוש אויף שופטים 12:6

Say Shiboles please. May I cross this river flow?¹Shiboles” is translated as “flow”.

Shibboleth (שִׁבֹּלֶת) – Just as they would test them with this word, so too they would test them with ever word that has a שׁ (shin) in it, and the Ephrathites would read it as שׂ (sin), and the text chose one of the words. And some say that he chose the word "שבולת" (shibboleth) because there was a flowing stream (shibboleth nahar) in the place where the Ephraimites were crossing, who would stutter on the letter שׁ (shin). Maybe the climate of their land was a factor! This is like the people of France, who do not understand how to read שׁ (shin) and read it like a weak תֿ (thaw).

They were not proficient in proper pronunciation. They were speech deficient.