באַלסק
'Bielsk Podlaski (town in eastern Poland)

ETYMOLOGY

Beider writes that the Yiddish name was likely derived from ✱Baylsk, where the original diphthong was monophthongized before the consonant cluster. He suggests that the hypothetical diphthong arose due to "anticipatory softening" before a palatalized consonant.
Polish Bielsk.

BALSK [Yiddishland, Paul Glasser]