The Collaborative International Dictionary of English v.0.48:

Fortissimo \For*tis"si*mo\ (? or ?), adv. [It., superl. of
   forte, adv. See Forte, adv.] (Mus.)
   Very loud; with the utmost strength or loudness.
   [1913 Webster]

WordNet (r) 3.0 (2006):

fortissimo
    adv 1: a direction in music; to be played very loudly [syn:
           fortissimo, very loudly] [ant: pianissimo, very
           softly]
    adj 1: chiefly a direction or description in music
    n 1: (music) loud [syn: forte, fortissimo]