Cite the scanned version of the original dicionary like this:
Toller, T. Northcote, and Joseph Bosworth. "níd-mǽg." An Anglo-Saxon Dictionary: Based on the Manuscript Collections of the Late Joseph Bosworth : Supplement. Oxford: Clarendon Press, 1921. 65.
, es; m. A near kinsman, a cousin :-- Iohannes úres Dritenes nýdmǽg, L. Ælfc. P. 9; Th. ii. 366, 37. v. níd-máge, -freónd, -sibb; and cf. Icel. nauð-maðr a near kinsman.