For information on Jewish names you could start with the Wikipedia article -
http://en.wikipedia.org/wiki/Jewish_names
Einmaldank appears to be a Yiddish rather than a German surname; by which I mean it doesn't appear to have a history among the surnames of non-Jewish German speakers. I would hazard a guess that it started out as a nickname based on some incident in the life of the original bearer. Alternatively it might be a nickname from an expression the original bearer used constantly.