What is Defendemus?

Legal Definition
Lat. A word used in grants and donations, which binds the donor and his heirs to defend the donee, if any one go about to lay any incumbrance on the thing given other than what is contained in the deed of donation. Bract. 1. 2, c. 16.
-- Black's Law Dictionary
Legal Definition
We will defend.
-- Ballentine's Law Dictionary