Soccer” comes from “association football.” In late-19th-century British school slang, people often shortened words and added “-er” endings. So “association” became “assoc”, then “soccer.”

It was originally a British term, not an American one. In the UK, it used to be common, but over time “football” became the usual word there, while “soccer” stayed common in places like the U.S., Canada, Australia, and New Zealand.