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1. In baseball, a ball that curves in its trajectory toward home plate. He's a good batter, but he has a hard time hitting a curve ball.
2. An unexpected occurrence or thing that causes confusion or uncertainty. I thought I had confirmed my hypothesis, so these results are a real curve ball.
3. A deceptive action or thing. I don't know, I think her sudden displays of affection are just a curve ball to get me to trust her.
throw (one) a curveball
To introduce a problem piece of information that shocks, bewilders, or confounds one or makes it more difficult for one to succeed. An allusion to baseball, in which a curveball is a pitch that arcs toward the batter in order to be more difficult to hit. Primarily heard in US. The end of that trick always throws the audience a curveball. I love watching their faces as they desperately try to figure it out. It really threw Stu a curveball when Olivia announced she was leaving the company.
throw somebody a ˈcurveball(American English) surprise somebody with a problem, situation, question, etc. that they do not expect and which is difficult to deal with: Just when you think you have it all under control, life throws you a curve ball.
In baseball, a curveball is a ball that is difficult to hit because it does not move in a straight line.