In the families Thesing, that I researched, the oldest form of the name is "Theissinck". And Theis is short for Mattheis, which is Mattheus or Matthew. The ending "-ing" in Germanic names means "son of". So Thesing means: "son of Matthew." On the pronunciation of the name: Thesing is pronounced as "Taising" in the Netherlands and as "Taizing" in Germany.
Have you already found out which Thesing family you belong to? I have researched the families in Herbrum and Dörpen in the German Emsland. These families are Roman Catholic. The Emsland is in the Northwest of Germany, just east of the Dutch border.