I think most musicians get the idea when they see two Fs together, and when there's a Z involved, it usually means to smack the hell out of something.
ffz is an abbreviation for "forzando" or "forzato". It basically means to play the note at the indicated dynamic (ff in this case) but with emphasis, as though it had a strong accent mark over it.
Now, this term can be confused with two others:
sffz is an abbreviation for "sforzato", which is to play at the dynamic level written (ff) and with a strong accent and at full duration, as below.
sff is an abbreviation for "sforzando", which is to play at the dynamic level written (again, ff) but with a typical accent mark, as shown below.
So just remember -- if it's got a Z, it means it's a strong accent (Z looks kind of like a backwards S, and even the sound Z makes sounds stronger than an S, right?). If it's got an S and a Z, it means to play a long, strong accent (because the abbreviation is longer, and has that Z again). If it's just got an S, it means to play it like a regular accent (strong, but not as strong as a Z).