A Cager is simply a basketball player....

When the game was first invented by James Naismith, there weren't out of bounds. The rules stated the first player who got the ball could throw it back in. We could only imagine how heated some skirmishes would become, with some even involving fans. Hometown fans could beat on opposing players and fights would break out in the middle of stands.  

A cage (sometimes 12 feet in height) was installed around the court as a safety measure during the early days of the game. This ultimately helped protect the audience and the players from any physical altercations. As a result, Basketball games were called Cage Games and the players were known as Cagers. Cages were still used around the court as late as 1925.

Until recently, Cager was a common name often used in newspaper articles.  It has mostly disappeared, but the name still makes an appearance from time to time.

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Sports Illustrated Vault