The first line of Isaac Bashevis Singer's Satan in Goray goes:

In the year 1648, the wicked Ukrainian hetman, Bogdan Chmelnicki, and his followers besieged the city of Zamosć but could not take it, because it was strongly fortified; the rebelling haidamak peasants moved on to spread havoc in Tomaszów, Bilgoraj, Kraśnik, Turbin, Frampol—and in Goray, too, the town that lay in the midst of the hills at the end of the world.
_{Satan in Goray, chapter 1: "The Year 1648 in Goray" (no emphasis added)}

I'm not sure what the italicized word "haidamak" is supposed to mean here. What language is it in? Is it Yiddish (the language the book was originally written in)? Is it Polish? What does it mean? And why was it left untranslated here?