The word soccer comes from association football, the formal name used when the rules of the game were standardized in Britain. At the time, several kinds of football existed, so people needed a clear way to tell them apart.
One of the main contrasts was this:
- association football, which became soccer
- rugby football, which became rugger in older British slang
In the late 1800s, British students often made playful shortened forms of words and added an er sound. Association was shortened to assoc., which was pronounced more like soc or socca. From there, soccer developed as a casual nickname.
This means the word was originally British, even though many people now think of it as mainly American. In countries where football already clearly means the association game, people usually just say football. In places where another sport is also called football, such as the United States, soccer stayed useful because it avoids confusion.
So soccer is not a modern mistake. It is a historical nickname that grew out of association football.