… is an arcane nickname for basketball players coming from the days when, believe it or not, they actually played in a cage.
Just like they do in modern-day Macau.
According to wiki.answers.com:
In the initial era of basketball as we know it, an out of bounds pass was awarded to the first person to possess it. Well, it didn’t take long for this rule to cause problems. Players were fighting for balls off of the court…fans were getting involved…and it was more like today’s pro wrestling than it was basketball. Well, like hockey, it was decided that it would be an improvement for fans if the court was “caged” with a fence, and the patrons were protected. The players eneded up playing in a “cage,” and thus the tag “Cagers” became a common term of endearment. Soon after the decision to “cage” the court came an even better one – to change the out of bounds rule altogether.
Once the rule was changed, the fence was removed, and the “cagers” became a thing of the past. Now you know.