Zadek
I believe it is German, possibly Jewish.
Replies
Zadek is Jewish (see http://www.avotaynu.com/books/MenkNames.htm).
According to the Oxford Names Companion this name is from the Hebrew tsadik meaning a pious or saintly man.
In their Jewish Family Names and Their Origins, Guggenheimer & Guggenheimer give the origin of Zadek (also Zad(d)ik, Tsad(d)ik, Sad(d)ik) as either Hebrew 'just, justified' or Arabic 'true, truthful, sincere'.
Arabic if written with S