In American English a curveball is a deceptive or unexpected action which poses a new challenge e.g. He threw a curveball by threatening to walk out of the meeting.
The term comes from baseball and refers to a ball pitched to curve away from the hitter.
There are many other examples of baseball terminology entering general English - three strikes and your out, hit a home run, touch base. Interestingly these are commonly used in British English while cricket expressions sticky wicket, a good innings are unfamiliar to most Americans.
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