pi·a·nis·si·mo 
(pē′ə-nĭsə-mō′) Music
(pē′ə-nĭsə-mō′) Music adv. & adj. Abbr. pp
In a very soft or quiet tone. Used chiefly as a direction.
n. pl. pi·a·nis·si·mos
A part of a composition played very softly or quietly.
[Italian, superlative of piano, soft; see PIANO2.]