An Etymological Dictionary of the German Language/Annotated/Ketzer
Ketzer, masculine, ‘heretic,’ from Middle High German ketzer, masculine, ‘heretic,’ also ‘reprobate, Sodomite’ (not recorded in Old High German). The tz presents no difficulties in deriving the word from Greek καθαρός (καθαροί, a Manichean sect spread throughout the West in the 11th and 12th centuries, and persecuted by the Church), if it be assumed that Dutch ketter, ‘heretic,’ is a phonetic version of the High German word. It is true that High German tz from Greek θ (Latin th) cannot be demonstrated; the hard fricative th (þ, θ) may, however, be regarded phonetically as tz, since, e.g., King Chilperic's sign for the was none other than z; the þ in Old Icelandic words sounded also to the Germans of the 9th century like z; þór seemed to them zor. So too in Italy the καθαροί were called Gazari.