A person who performs cartwheels, or someone skilled at acrobatic movements involving rotating sideways on their hands and feet.
From 'cartwheel' (a gymnastic move named for its resemblance to a spinning wheel), plus the agent suffix '-er.' The cartwheel move and the word emerged in the 19th-20th century as gymnastics and acrobatics became popular entertainment.
Cartwheel terminology is a funny example of how sports borrow names from everyday objects—when gymnasts saw wheels on carts, they named their athletic move after them, and now most people know 'cartwheel' better as an acrobatic move than as actual wheel rotation!
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