The story
gym (short for gymnastics) + shorts
Etymology from Wiktionary, CC BY-SA 4.0.
Today it means: A type of relatively lightweight shorts with elastic waistbands designed for use in sports and physical activity, especially gym class. The terms basketball shorts and mesh shorts are used nearly synonymously with gym shorts in the United States, despite their different initial usage. Full definition →
gym (short for gymnastics) + shorts
Etymology from Wiktionary, CC BY-SA 4.0.