The Collaborative International Dictionary of English v.0.48:

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.
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            His fierce thunder drove us to the deep. --Milton.
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   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.
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            The fierce foe hung upon our broken rear. --Milton.
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            Thou huntest me as a fierce lion.     --Job. x. 16.
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   3. Excessively earnest, eager, or ardent.

   Syn: Ferocious; savage; cruel; vehement; impetuous;
        barbarous; fell. See Ferocious. -- Fierce"ly, adv.
        -- Fierce"ness, n.
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WordNet (r) 3.0 (2006):

fiercely
    adv 1: in a physically fierce manner; "silence broken by dogs
           barking ferociously"; "they fought fiercely" [syn:
           ferociously, fiercely]
    2: in an emotionally fierce manner; "she was fiercely proud of
       her children"