What is the source of the name Isser? On what Yiddish, Hebrew or Jewish term - if any - is it based?
This book explains that Isser is a variant of the name Yisrael, citing a tradition of dropping God's name from the name itself - like Eliyahu becoming Elya, for example.
As per the sefer Zocher Ha'bris (24-17) and Sefer Ha'bris (page 318-22) it appears that the name Isser is actually not a variant of Yisroel but it is pshuto ki'mashma'o and it means "prohibition." Apparently whenever an unmarried girl gave birth to a boy, the newborn was named Issur. Maybe later on, in order to whitewash the family history, the idea of it being a variant of Yisroel was created.