Although soy sauce comes originally from China, our word for the sauce comes from the Japanese word, shoyu. This has nothing to do with the actual Japanese word for soybeans themselves, which is daizu. The Chinese word for soy sauce, on the other hand, is jiàngyóu, while the word for the beans is dàdòu.
So, our English word soy does not come from the word for the beans themselves, but from the Japanese word for the sauce, which first appeared in Japan sometime in the 1500’s.
The origin of the English word soy for the sauce seems to have come from philosopher John Locke, writing in his journal in 1679. He talked about two sauces that were the most common types of sauces coming from the East Indies, mango and shoyu. He rendered the word shoyu as saio, which went on to be simplified to soya or soy by later written references.
You can listen to the pronunciation of shoyu here:
Soy sauce actually was the first fermented bean (legume) product that was introduced to Europe from Asia and is still the most widespread fermented legume product on Earth.
Although modern soy sauce making is a highly refined and scientific process, and it is not known exactly how it was made in the earliest times, it is thought that the liquid was simply a byproduct of the making of fermented soybean pastes such as miso, or the Chinese ancestor of miso, chiang. A dark liquid would pool at the top of the fermenting vats which would be skimmed off and used as a sauce. Over time, processes were developed to make this liquid sauce specifically. Even today, the making of soy sauce is a bit different than the making of miso, which results in soy sauce having a more complex, sharper and stronger flavor that miso paste.