An Etymological Dictionary of the German Language/Annotated/Mauer
Mauer, feminine, from the equivalent Middle High German mûre, mûr, singular, ‘wall,’ Old High German mûra, feminine (mûri, feminine), ‘wall,’ from Latin mûrus, with an abnormal change of gender, which is probably caused by an Old Teutonic word for ‘wall’; compare Gothic waddjus, feminine. At the same period, before the High German permutation, Old Saxon mûr, Anglo-Saxon mûr, masculine (Old Irish múr), were also borrowed from the Latin, like other words relating to stone buildings; compare Siegel, Fenster, Pforte, Speicher, &c.