Source : Webster's Revised Unabridged Dictionary (1913)

Fortissimo \For*tis"si*mo\ (? or ?), adv. [It., superl. of
   forte, adv. See {Forte}, adv.] (Mus.)
   Very loud; with the utmost strength or loudness.

Source : WordNet®

fortissimo
     adj : chiefly a direction or description in music
     n : (music) with great loudness [syn: {forte}]
     adv : a direction in music; to be played very loudly [syn: {very
           loudly}] [ant: {pianissimo}]