An Etymological Dictionary of the German Language/Annotated/Mahr
Mahr, masculine, from the equivalent Middle High German mar, mare, masculine and feminine, ‘incubus, nightmare,’ Old High German mara, feminine; compare Anglo-Saxon mara, masculine, English mare in nightmare, Old Icelandic mara, feminine, ‘incubus.’ The derivation from Gothic marzjan, ‘to vex,’ Old High German marren, męrren, ‘to hinder, disturb,’ is scarcely possible. Some etymologists connect the word with Slavonic-Russian kikimora, ‘ghost,’ Polish mora, Bohemian můro, ‘nightmare.’ From Mahr, French cauchemar, ‘nightmare,’ has also been derived (caucher, from Italian calcare, ‘to tread, press’).