Someone who plays ball sports, or slang for a successful, impressive person who is good at what they do.
From 'ball' (sphere used in sports) + '-er' (one who does). The slang sense meaning 'successful person' emerged in African-American Vernacular English around the 1990s, popularized by hip-hop culture.
The slang meaning spread from sports to mean anyone successful and impressive—it shows how African-American Vernacular English creates vivid, efficient words that eventually go mainstream. 'Baller' is now used in business and academics, far from its basketball origins!
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