fortississimo

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.

Fun fact: For every 'f' or 'p' added beyond the second, you add another 'iss' to the name.

Frex:

   f: forte
   ff: fortissimo
   fff: fortississimo
   ffff: fortissississimo
   fffff: fortississississimo

...and so on. Impress your band director. It should be noted that there is almost never anything louder than fortississimo, but should you come across something that is, you now know what to call it.