the Polish province of Mazovia (?) :-- Be norþan Horiti is Mægþa land; and be norþan Mægþa londe Sermende óþ ða beorgas Riffen, Ors. l, l; Swt. 16, 21.
Cite the scanned version of the original dicionary like this:
Bosworth, Joseph. "Mægþa land." An Anglo-Saxon Dictionary: Based on the Manuscript Collections of the Late Joseph Bosworth. Ed. Thomas Northcote Toller. Oxford: Clarendon Press, 1898. 658.