Ottomans were invented around 18th century in Turkey, when the country was ruled by the Ottoman Empire. Back then, an ottoman looked very different than what we use to see: it was a low piece of upholstered furniture with no arms that was piled with cushions.
It was delightfully-named “pouf” (French) when it made its way to Europe later on.
As time went on, ottomans & puffs evolved to become smaller and freestanding, most often used as extra seating scattered around a room. They often call it “overstuffed”: meaning that no wood other than the legs is visible.