An Etymological Dictionary of the German Language/Annotated/Kerker
Kerker, masculine, ‘gaol,’ from the equivalent Middle High German karkœre, kęrkœre, kęrker, masculine, Old High German karkâri, masculine, ‘prison'; from Latin carcerem, probably more strictly from karkerim (compare Old High German krûzi, from Latin crucem under Kreuz), so that the final i of the Old High German word would represent the -em of the accusative (compare Kelch, Linse). Even in Gothic karkara, feminine, ‘prison,’ is found, corresponding to Anglo-Saxon cearcern, Old Irish carcar. In the High German word the second k shows that Kerker was borrowed before the Old High German period, since borrowed terms in Old High German such as chrûzi, from crucem, pronounce the c as tz before open vowels; compare Kaiser, Kelch, Keller, Kerbel, and Pech.