FORTE, loud: an Italien word, usually abbreviated into f. A lesser degree of loudness is expressed by mf–mezzoforte; a greater one by più f and ff, and the greatest of all by fff–fortissimo, as in Beethoven's 7th Symphony (Finale), 8th ditto (1st movement), Overture, op. 115 (at end), Leonore, No. 2 (8vo score, pp. 40, 76), or at the grand elimax near the close of the Finale of Schubert's Symphony in C, at the end of the extraordinary long crescendo.
fffff has been occasionally used by later composers, as in the Overture to 'Charlotte Corday,' by Benoit. [Grove Dictionary 1879, 556]