The Collaborative International Dictionary of English v.0.48:

Pianissimo \Pi`a*nis"si*mo\, a. [It., superl. of piano.] (Mus.)
   Very soft; -- a direction to execute a passage as softly as
   possible. (Abbrev. pp.)
   [1913 Webster]

WordNet (r) 3.0 (2006):

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

Moby Thesaurus II by Grady Ward, 1.0:

86 Moby Thesaurus words for "pianissimo":
   a cappella, a la sourdine, abbandono, accrescendo, adagietto,
   adagio, affettuoso, agilmente, agitato, allegretto, allegro,
   amabile, amoroso, andante, andantino, appassionatamente,
   appassionato, barely audible, brillante, capriccioso, con affetto,
   con agilita, con agitazione, con amore, crescendo, decrescendo,
   dim, diminuendo, dimly, distant, dolce, faint, faint-voiced,
   faintly, feeble, feebly, forte, fortissimo, gentle, gently,
   half-heard, hushedly, indistinct, lamentabile, larghetto,
   larghissimo, largo, legato, leggiero, low, marcando, morendo,
   murmured, parlando, piano, pizzicato, prestissimo, presto,
   rallentando, ritardando, ritenuto, scarcely heard, scherzando,
   scherzo, scherzoso, soft, soft-sounding, soft-voiced, softly,
   sordamente, sordo, sotto voce, spiccato, staccato, stretto,
   subaudible, subdued, subduedly, tremolando, tremoloso, trillando,
   unclear, weak, weak-voiced, weakly, whispered