The story
Phono-semantic matching of Japanese ソフトテニス (sofuto tenisu), itself a wasei eigo (和製英語; pseudo-anglicism) derived from soft + tenis.
Etymology from Wiktionary, CC BY-SA 4.0.
soft tennis
Today it means: A variant of tennis, developed in Japan, differing in the tennis ball used, from regular tennis (lawn tennis), having a softer rubber ball. Full definition →