Cite the scanned version of the original dicionary like this:
Toller, T. Northcote, and Joseph Bosworth. "un-." An Anglo-Saxon Dictionary: Based on the Manuscript Collections of the Late Joseph Bosworth : Supplement. Oxford: Clarendon Press, 1921. 732.
.The prefix (1) expresses negation; (2) gives a bad sense, as in un-dǽd, un-dóm, un-lagu, un-lǽce; (3) reverses an action, as in un-bindan, un-dón; (4) is intensive(?) v. un-hár.