Usual English form of Dorothea. The name was not used in the Middle Ages, but was taken up in the 15th century and became common thereafter. It was borne by the American film star Dorothy Lamour (1914–1996, born Dorothy Kaumeyer).
French: from the Old French oblique case of a Germanic personal name, originally probably a byname from gasti ‘stranger’, ‘guest’, ‘host’. Compare Guest. The surname is also found in England and Ireland, where it is probably a Huguenot importation.