Un-síþ

According to the Old English Dictionary:

es;

un-síþ

m. I. an evil, ill-advised expedition :-- Wearð ofslegen Ecgfridus on his unsíðe, ðá ðá hé on Peohtum begann tó feohtanne tó dyrstelíce ofer Drihtnes willan (cf. Ecgfrid, cum temere exercitum ad vastandam Pictorum provinciam duxisset, multum prohibentibus amicis, extinctus est, Bd. 4, 26), Homl. Th. ii. 148, 16. II. a mishap, misfortune :-- Heó ðurh wódnysse micclum wæs gedreht .. beó ǽr ðon eáwfæst leofode, ðeáh ðe se unsíð hire swá gelumpe, Homl. Th. ii. 142, 12. [Þu (the owl) ne singst never þat hit nis for sume unsiþe (mishap), O. and N. 1164.] un-siþ