When you're playing off some sheet music on a piano, and it tells you to play piano, any musician can tell you that it means quietly, not (quite redundantly) on the instrument you're already using. So it would be a logical conjecture to conclude that the piano instrument comes from the level of volume piano, right? Well, kind of. It actually comes from the Italian word pianoforte, which meant "soft yet strong", because of the large range of sounds a piano can make. The "soft" meaning is indeed in the prefix part, piano, but now it is evident that that is not where the word comes from directly. Through Latin planus, "even", piano traces to the Proto-Indo-European term pele, "flat", which is also surprising, but makes sense when you consider that the term just got more figurative over time. Meanwhile, forte, another measure of sound in music, meant "loud" or "strong", also got more figurative through the ages, developing from Latin fortis, which meant "fortress", from PIE bergh, "to rise". So pianoforte, and by extension, a piano, really means "rising flat" or "even fortress". Music is so complicated.
Adam Aleksic, a rising sophomore studying linguistics and government at Harvard University, has been described as the internet's sixth most famous etymologist. He also has disturbing interests in politics, vexillology, geography, board games, conlanging, and law.