'Fierceness' definitions:

Definition of 'fierceness'

noun

The property of being wild or turbulent; "the storm's violence" [syn: ferocity, fierceness, furiousness, fury, vehemence, violence, wildness]

From: GCIDE

Fierce \Fierce\, a. [Compar. Fiercer; superl. Fiercest.] [OE. fers, fiers, OF. fier, nom. fiers, fierce, savage, cruel, F. fier proud, from L. ferus wild, savage, cruel; perh. akin to E. bear the animal. Cf. Feral, Ferocity.]
1. Furious; violent; unrestrained; impetuous; as, a fierce wind. [1913 Webster]
His fierce thunder drove us to the deep. --Milton. [1913 Webster]
2. Vehement in anger or cruelty; ready or eager to kill or injure; of a nature to inspire terror; ferocious. "A fierce whisper." --Dickens. "A fierce tyrant." --Pope. [1913 Webster]
The fierce foe hung upon our broken rear. --Milton. [1913 Webster]
Thou huntest me as a fierce lion. --Job. x. 16. [1913 Webster]
3. Excessively earnest, eager, or ardent.
Syn: Ferocious; savage; cruel; vehement; impetuous; barbarous; fell. See Ferocious. -- {Fierce"ly}, adv. -- {Fierce"ness}, n. [1913 Webster]

From: GCIDE

fierceness \fierceness\ n. the quality of being fierce; ferocity; fury; vehemence.
Syn: ferocity, furiousness, fury, vehemence, violence, wildness, strength. [WordNet 1.5 +PJC]