The first Mostert was a strong Dutchman who broke three swords in battle with the “heathen” during the 5th Crusade at the battle of Damietta (Egypt) in 1219. When the Emperor Frederick II heard of his feats, he is reported to have exclaimed that this soldier was “as strong as mustard’, mosterd in Dutch, mostrich in German. On being knighted by the Emperor, he took on that as his name.

Old spellings were Mostaert (the name of a family of great Duch painters, headed by Jan Mostert of The Haag) in the late 15th and early 16th century).