...is not derived from "Katze" (i.e. "cat") although that is how the Katzer family worldbook by Halbert's Family Heritage Inc. explains it. We rather believe that the name is derived from "Ketzer" ("a heretic"). This is also confirmed in the Middle German linguistic handbook (Mittelhochdeutsches Handwörterbuch) by Matthias Lexer. We have also been told that the Czech language has the word "kacir" to describe someone who is a heretic. This was confirmed by the Czech fellow students of Professor Georg Katzer, who once studied in Prague for some time. Another interesting aspect in this context is the meaning of the Czech place name of "Kraliky" (the former "Grulich"), which appears to be the cradle of the Katzer families.
Franz Gerhard Soural told us that his late aunt had mentioned in her family chronicle that the Czechs spelled the name "Kadcera"
Another interesting explanation is the alteration of vowel sound from "e" to "a", which was quite common in the Silesian dialect (e.g.das Herz = doas Harze or Klecks = Klacks). It would make the change from "Ketzer" to "Katzer" very likely
According to the German Brockhaus Encyclopedia
the word "Ketzer" has developed from the term "Katharer" (i.e.
Catharist). It referred to a heretic who had diverged from the
Catholic doctrine.
Cathari (Greek: "the pure ones") was the name of a religious
movement invading Europe from the Balkans at the beginning of the
12th century. They
quickly found supporters in the Rhineland, Northern and Southern France, England
and Northern Italy. Not much later the church initiated counteractions that
lead to bloody persecutions of the Cathari. "Ketzer" may, of course,
also refer to any outragous person. Katzers have obviously never been well
adapted people.
The theory of the name relationship ' Ketzer-Katzer' is also supported by a report in the historic chronicle "Neue Kronik von Böhmen" dated 1780 (page 116): 
The complete chronicle can be read in Google Books. The Book is only available in German, though.
A German dictionary of family names (Duden der Familiennamen) offers additional explanations:
- It could refer to Upper Sorbian. kacor, Lower Sorbian. kacor, Czech. kacer which means something like a drake.
- It could also refer to peoples' places of origin: There are placenames such as Unterkatz (Thuringia) or Katzen (East Prussia).
The same dictionary also mentions several derivations of the initial part of the name: "Katz":
- It could refer to the Middle High German term "katze". That was a military instrument of siege: A mobile shelter for the besiegers or slingshot users. (If your ancestors appear to have been warlike people this is what I would suggest ;-))
- As a Jewish family name, "Katz" could also have been formed by the contraction of the Hebrew terms "Kohen-zedek" (priest of justice).
- The term could also refer to a place of residence situated in an area with a homonymous field name.
- In Austria it could also have been derived from "Achatz" (Name of the holy martyr Achatius (2nd century), one of the fourteen helpers in need).