On the etymology of "soccer":

  1889, socca, later socker (1891), soccer (1895), originally university slang (with jocular formation -er (3)), from a shortened form of Assoc., abbreviation of association in Football Association (as opposed to Rugby football); compare rugger. An unusual method of formation, but those who did it perhaps shied away from making a name out of the first three letters of Assoc. Compare 1890s English schoolboy slang leccer, from lecture (n.).

Source: https://www.etymonline.com/word/soccer