Curve ball: a pitch thrown with a rotation that makes the ball curve.

曲线球：投出旋转球，使球的飞行线路呈曲线。

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At the last minute, I threw them a curve ball by saying, "We're going to bring spouses."

在最后一刻，我给了他们一个惊喜，说：“我们会带配偶。”

《柯林斯英汉双解大词典》

So as a ball slows down at the end of its trajectory, the curve becomes more pronounced.

所以当球运行到终点时，由于速度下降，将会划出更明显的曲线。

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But a person who threw a curve ball did something unexpected.

VOA: special.2009.04.12

Could the wider distribution of the latest iPhone have thrown a curve ball at HTC?

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"This was a very big curve ball, " said Jay Wheatley, head of the tax-exempt bond syndicate desk at Citigroup.

WSJ: Tax-Cut Deal Results in Turmoil for Bonds

The scary part was supposed to be tablet and smartphone market, where European conumer demand trouble is creating a curve ball for investors.

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