Latin, se defendere.
When Domesday Book records that an estate 'answers' - or 'defends itself' - for a certain number of hides, it is stating its assessment for the public obligations laid on all estates. 'Defending' land - 'answering' for it - normally involved paying geld, providing military service, and performing other services due from the land.
For more detail, see J.H. Round, Feudal England (1895); and Rosamond Faith, The English peasantry and the growth of lordship (1997).