In music notation, fortississimo is abbreviated 'fff' and indicates that the loudness of the music following it should be as loud as possible. The word means "very very loudly" in Italian.
When added to the staff, it looks something like this (complete with eigth notes):
/\
---| /----------------------------------------------------
|/
---/---------------*---------*---------*---------*--------
/| | | | |
-/-|/\--------*---|-----*---|-----*---|-----*---|---------
| | | | | | | | | | |
|--|--|------|----|----|----|----|----|----|----|---------
\ | | |____|____|____|____|____|____|____|
--\|_/----------------------------------------------------
|
\| fff
There is also a rarely-seen 'ffff' marking, which would be played even more loudly than fortississimo.