Definition of "curveball"

The season’s first episode, “Bart Gets an F,” got the highest ratings in its history, but “Treehouse of Horror” was the real curveball: a genre-busting effort that proudly punched above the show’s expectations.

2015 March 10, David Sims, “Without Sam Simon, 'The Simpsons' Wouldn't Be What It Is Today”, in The Atlantic‎

There are two curveball nominations, one perhaps a popularity bid and the other a credibility bid. The popular one is Bieber’s Purpose.

2016 December 6, Spencer Kornhaber, “The Culture Wars in the Grammy Album Nominations”, in The Atlantic‎

But from time to time, June throws up a curveball, in the shape of unusual and hard to predict meteorological events.

2017 June 13, Stephen Moss, “June likes to throw a curveball now and again”, in The Guardian‎

And I think that’s an example of where faced with a seemingly intractable political reality, technology produces a solution that is a total curveball and not that imaginable a few decades ago.

2021 June 11, Ezra Klein, “Sam Altman on the A.I. Revolution, Trillionaires and the Future of Political Power” (32:37 from the start), in The Ezra Klein Show‎ (podcast), spoken by Sam Altman

Even though the haughty physics professors at the elite school ridiculed his declaration that he could make a baseball curve, Cummings just laughed it off and said, “I curveballed them to death,” according to Frederick Ivor-Campbell.

2006, William F. McNeil, The Evolution of Pitching in Major League Baseball, McFarland, page 21

The news curveballed him. He'd been hamstrung and schizzed all the preceding weeks. He brooded in his den.

2009, James Ellroy, Blood's a Rover, Random House, page 566