1640s, fort, from French fort "strong point (of a sword blade)," earlier "fort, fortress" (see fort). Meaning "strong point of a person, that in which one excels," is from 1680s. Final -e- added 18c. in imitation of Italian forte "strong."
1767, from Italian, from piano e forte "soft and loud," in full, gravicembalo col piano e forte "harpsichord with soft and loud" (c. 1710), said to have been so named by inventor Bartolomeo Cristofori (1655-1731) of Padua because the ability via dampers to vary the tone is one of the main changes from the harpsichord. Italian piano (adj.) ultimately is from Latin planus "flat, smooth, even," later "soft" (from PIE root *pele- (2) "flat; to spread"). Also fortepiano.