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]

Moby Thesaurus II by Grady Ward, 1.0:

87 Moby Thesaurus words for "fortissimo":
   a cappella, abbandono, accrescendo, adagietto, adagio, affettuoso,
   agilmente, agitato, allegretto, allegro, aloud, amabile, amoroso,
   andante, andantino, appassionatamente, appassionato, booming,
   brillante, capriccioso, con affetto, con agilita, con agitazione,
   con amore, crescendo, deafening, decrescendo, diminuendo, dolce,
   ear-piercing, ear-rending, ear-splitting, earthshaking, forte,
   fortemente, full, in full cry, lamentabile, larghetto, larghissimo,
   largo, legato, leggiero, loud, loud-sounding, loudish, loudly,
   lustily, marcando, morendo, noisily, parlando, pealing, pianissimo,
   piano, piercing, pizzicato, plangent, prestissimo, presto,
   rallentando, resounding, resoundingly, ringing, ringingly,
   ritardando, ritenuto, scherzando, scherzo, scherzoso, sonorous,
   sordo, sotto voce, spiccato, staccato, stentoraphonic, stentorian,
   stentorious, stretto, thunderous, tonitruant, tonitruous,
   tremolando, tremoloso, trillando, uproariously, window-rattling