Cite the scanned version of the original dicionary like this:
Toller, T. Northcote, and Joseph Bosworth. "tácnung." An Anglo-Saxon Dictionary: Based on the Manuscript Collections of the Late Joseph Bosworth : Supplement. Oxford: Clarendon Press, 1921. 72.
, e; f. Signification :-- Tácnunga significationem, Ps. Spl. 59. 4. I.an indication, sign, characteristic mark, symptom :-- Lǽcedómas and tácnung on ðam roppe (cf. be tácnum on ðam roppe, 230, 16), Lchdm. ii. 164, 5. Be lyfte tácnungum de aeris indiciis, Nar. 3, 14. Hit nú is búton swylcum tácnungum ðæs yfeles ðe hit ǽr dyde Aetna nunc tantum innoxia specie ad praeteritorum fidem fumat, Ors. 2, 6; Swt. 90, 3.