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.