The upcoming documentary, Sly, reveals the origin story behind his character's name in Rocky. Sylvester Stallone rose from relative obscurity by writing and starring in the 1976 film Rocky as the titular hard-nosed club fighter from Philadelphia who gets a once-in-a-lifetime shot at the world heavyweight championship. Rocky, which became a massive critical and commercial success, launched Stallone's career as an action star and the upcoming documentary Sly will provide an intimate look at his prolific career.
https://screenrant.com/rocky-balboa-name-inspiration-sylvester-stallone/