Soccer

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late 19th century: shortening of Assoc. + -er4.

wiktionary

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British English; Colloquial abbreviation for  association football, via abbreviation  assoc. +‎  -er(slang suffix); earlier  socker (1885), also  socca (1889), with  soccer attested 1888. 

Compare contemporary rugger, from Rugby, and note vulgar connotations of analogous *asser if abbreviating on first syllable. [1] Similarly constructed coinages from the same period include: brekker(“breakfast”), fresher(“freshman”) and footer(“football”). See Oxford -er.

etymonline

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soccer (n.)

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.).