English: probably from an otherwise unrecorded Old English personal name, cognate with the attested Continental Germanic form Timmo. This is of uncertain origin, perhaps a short form of Dietmar. The personal name Timothy was not in use in England until Tudor times, and is therefore not a likely source of this surname, which is medieval in origin.
North German and Dutch: from a short form of the medieval personal name Dietmar.
Dictionary of American Family Names, Oxford University Press, Mar 2003