Dorsen means to thresh. The person doing the threshing is the dorser. You will not often find that as an occupation as it was just one of many jobs farms or their laborers did.

Threshing day in Zeddam, 1919. Credits: Bernard F. Eilers, collection Amsterdam City Archives (public domain)
Hi Yvette,
re: Baarle-Nassau/Hertog
I read that the criteria user by the Duke of Brabant as to which lands passed to his vassal was that he kept those lands that were paying him a rent and made the other lands over to his vassal.