The piano is also known as a
pianoforte. That was, in fact, the original name, but it got shortened to piano over time, which is actually a bit silly, if you consider the reason the pianoforte got it's name.
The pianoforte got it's name from the Italian words piano, which means soft, and forte, which means strong. Until the invention of the piano, instruments similar to it did not have varying loudness. The harpsichord was the predominant harmonic instrument before the piano's invention, and it is impossible to vary the loudness of the notes. The came the piano! Wow! One can now play loud OR soft! It was a very exciting time for musicians.
And now it's just called piano because of pesky neighbours.