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."
music instruction, "loud, loudly," from Italian forte, literally "strong," from Latin fortis "strong" (see fort). Opposed to piano.