The word "erdergbassor" does not appear in any Hungarian dictionary.
However, the word for "witch" in Hungarian is "boszorkány", the first two syllables of which would be pronounced something like the second half of the term in question: "bassor". The first part of the term — "erderg" — is more mysterious. Fortunately, there is a hint in the fact that Steve deliberately transliterated "bűz" as "Burz"; he clearly intends to emphasize that the stressed vowels of Hungarian are implicitly strongly rhotic or r-colored in Fenarian. Given that, it seems reasonable to infer that "erderg" is most likely meant to be "ördög", which is Hungarian for "devil". This would make the entire term more properly written as "ördögboszor", and would mean something more literally like "devil-witch".