An Etymological Dictionary of the German Language/Ketzer
Ketzer, m., ‘heretic,’ from MidHG. ketzer, m., ‘heretic,’ also ‘reprobate, Sodomite’ (not recorded in OHG.). The tz presents no difficulties in deriving the word from Gr. καθαρός (καθαροί, a Manichean sect spread throughout the West in the 11th and 12th cents., and persecuted by the Church), if it be assumed that Du. ketter, ‘heretic,’ is a phonetic version of the HG. word. It is true that HG. tz from Gr. θ (Lat. 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 OIc. words sounded also to the Germans of the 9th cent. like z; þór seemed to them zor. So too in Italy the καθαροί were called Gazari.