Historically, sushi was originally a form of fish that had been salted and fermented for preservation without refrigeration. The fish was sometimes packed with rice or rice mixed with vinegar in order to speed the fermentation process. In the Edo period, this was also known as hayazushi (早鮨, 早鮓) when packed in layers, pressed, and fermented for just one night. Something similar is still prepared today as narezushi (馴鮨, 熟鮨) that relies on fermentation with no added vinegar.
The kanji spellings for sushi are ateji.
すし (romaji sushi)