The story
Originally British English; as an abbreviation for association football, via abbreviation assoc. + -er (suffix); earlier socker (1885), also socca (1889), with soccer attested 1888. Compare contemporary rugger, from rugby. Similarly constructed coinages from the same period include: brekker (“breakfast”), fresher (“freshman”) and footer (“football”). See Oxford -er.
Etymology from Wiktionary, CC BY-SA 4.0.