Firnum

According to the Old English Dictionary:

fyrnum;

firnum

adv. [dat. or inst. pl. of firen a sin, crime] Fearfully, intensely; formīdŏlōse, immānĭter :-- Nǽre firnum ðæs deóp merestreám the sea-stream would not be so fearfully deep, Cd. 39; Th. 51, 26; Gen. 832. firnum