In older German dictionaries of the family names we can find the etymology of Tolkien! Today I have read Max Gotschald's Deutsche Namenkunde: unsere Familiennamen nach ihrer Entste hung und Bedeutung (J. F. Lehmanns Verlag, München/Berlin, 1942, p. 463) where we can find:
Toll|kiehn, -kühn wohl pruss. O.N.
It means: "perhaps Old Prussian place-name''.
I wonder which German dictionary was J.R.R. Tolkien's source of the information about the etymology from German adjective tollkühn.