In baseball, a pitch that curves through the air; figuratively, an unexpected or deceptive situation.
A compound of 'curve' (from Latin 'curvus,' bent) and 'ball.' The baseball term originated in the late 1800s when pitchers discovered they could make a ball curve, and the figurative meaning came from that baseball innovation.
The curveball wasn't proven to exist until 1875, when photographs finally showed that the pitch actually curves—before that, batters swore it was optical illusion! This led to 'curveball' becoming slang for any unexpected trick or deception.
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