According to the Dictionary of American Family Names (Oxford University Press, ISBN 0-19-508137-4)

Ehemann is of South German origin. It is a byname for someone under feudal obligations of some particular kind, from Middle High German e ‘law’,‘contract’ + man ‘man’.Jewish: (Ashkenazic). It is also thought to be a nickname from modern German Ehemann ‘husband’ (the word Ehe having progressively become restricted to the marriage contract and then to the state of matrimony itself). At one time in the Austrian Empire, only one son in a Jewish family was officially permitted to marry and start a family of his own: this may have been a surname adopted by such a person.