The story
From spreader (“a spacer or device for keeping two objects apart”) + pants, so-called because the device spreads the wearer's legs apart.
Etymology from Wiktionary, CC BY-SA 4.0.
spreader pants
spread·er pantsToday it means: A thick, firm but yielding device worn around the crotch—usually supported by straps above the shoulders—that spreads the wearer's legs apart, forcing them to crawl. Full definition →