An Etymological Dictionary of the German Language/Kerker
Kerker, m., ‘gaol,’ from the equiv. MidHG. karkœre, kęrkœre, kęrker, m., OHG. karkâri, m., ‘prison'; from Lat. carcerem, probably more strictly from karkerim (comp. OHG. krûzi, from Lat. crucem under Kreuz), so that the final i of the OHG. word would represent the -em of the acc. (comp. Kelch, Linse). Even in Goth. karkara, f., ‘prison,’ is found, corresponding to AS. cearcern, OIr. carcar. In the HG. word the second k shows that Kerker was borrowed before the OHG. period, since borrowed terms in OHG. such as chrûzi, from crucem, pronounce the c as tz before open vowels; comp. Kaiser, Kelch, Keller, Kerbel, and Pech.