"percussion musical instrument in which tones are produced by blows of hammers upon stretched strings, the hammers being operated from a keyboard," 1803, from French piano (18c.), Italian piano, shortened forms of pianoforte (q.v.).
Essentially, the pianoforte is a large dulcimer with a keyboard ; but historically it replaced the clavichord and harpsichord, which were keyboard-instruments more akin to the harp than to the dulcimer. [Century Dictionary]
Piano wire "kind of strong steel wire used for strings of pianos," is attested from 1831. Piano-case "wooden box enclosing the mechanism of a piano" is by 1844.
musical instruction, "softly, with little force or loudness," 1680s, from Italian piano, which is ultimately is from Latin planus "flat, smooth, even," later "soft" (from PIE root *pele- (2) "flat; to spread").