From Proto-Germanic *mann-, whence also Old English mann, Old High German man.
Descendants Old English: mann, man; manna English: man Old Frisian: man, mon West Frisian: man Old Saxon: man Old Dutch: man Dutch: man, men Afrikaans: man Old High German: man German: Mann, man Old Norse: maðr Icelandic: maður Faroese: maður Norwegian: mann Old Swedish: maþer, mander, man Swedish: man Old Danish: man Danish: mand Gothic: (manna)
Old Norse must have undergone some kind of sound change, otherwise the "d" would be remained in other branches, including English.
Or it is just an element form another Old Norse word? Moreover, the "r" is also unknown to me.
Is there any other example?