A basketball player, or a fighter who competes in a cage or ring.
From 'cage,' with the agent suffix '-er' creating a noun for one who participates in cage fighting or, metaphorically, basketball (which has a 'cage-like' court boundary).
The term 'cager' for basketball players became popular in the 1930s-40s when early basketball courts had actual wire cages around them to protect spectators from wild play!
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