The jock strap that protected the manhood of boxing legend Joe Frazier 45 years ago when he beat the GOAT, Muhammad Aliufeff, is about to hit the auction block ... TMZ Sports has learned.
The jock dates back to Ali-Frazier I in 1971 ... known as, "The Fight of the Century," one of America's most historic sporting events. Frazier -- who, like Ali, was undefeated -- won by unanimous decision.
Frazier gave the jock to his buddy the night of the fight ... who held onto it for almost 40 years before selling it to a memorabilia dealer in 2009.