very loud – used esp. as a direction in music

Pronunciation:
for-TISS-eh-moh

In a sentence:
Marianne’s post-supper solo was to be played fortissimo, so that everyone in attendance could hear what a prodigy she was, and know what a good job her mommy and daddy had done. 

In a different sentence:
Billy instructed his students to perform their recital pieces fortissimo and with passion, as he sat backstage with his headphones on and Rod Stewart in his cassette player.