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]