An Etymological Dictionary of the German Language/Annotated/Kreuz
Kreuz, neuter, ‘cross,’ from the equivalent Middle High German kriuz, kriuze, neuter, Old High German chrûzi, neuter; from Latin crŭci- (dative cruci, accusative crucem), with change of vowel quantity in the stem as in Leier, Lilie, and Litze, and also of gender (compare Abt, Orden, and Pech). The change of medial c in the Latin word to High German tz, though in another group of (older) loan-words Latin c, even before open vowels, appears as k in High German and Teutonic, is due to the fact that words like Keller and Kaiser were introduced into Germany at a far earlier period than Kreuz, which was adopted with Christianity in the 8th and 9th centuries. The Goths used Teutonic Galgen (Gothic galga), the English of the earliest period, rood (compare Rute). The loanword is now found in all the Modern Teutonic languages — Icelandic kross, Swedish and Danish kors, Dutch kruis, English cross.