Green’s Dictionary of Slang

(US) a tricky or unexpected question or action.

1945		‘G.I. Lingo’ AS XX:2 147: curved balls. tricky questions in an examination.
1951		C. Stoker Thicker ’n Thieves 206: I determined that, no matter what kind of a curve ball would be pitched, I would not be caught with my bat on my shoulder, and I wouldn’t be surprised by the pitch.
1967		W. Crawford Gresham’s War 32: He never came on like this before. He was aces up north [...] These were curve balls [HDAS].
1985		E. Bogosian Talk Radio (1989) 36: What’s the problem tonight, Stu? You’re throwing me some real curveballs, man.
1991		B. Hamper Rivethead (1992) 105: I was eventually reeled back into the GM nest. However, this time, a big curveball was awaiting me.
2001		(con. 1964–8) J. Ellroy Cold Six Thousand 253: He threw her a curveball — the Teamsters need help.
2001		D. Looser Boobslang [U. Canterbury D.Phil. thesis] 51/1: curveball n. a diversion.
2010		K. Richards Life 530: I felt that I should take her seriously. Chrissy doesn’t throw you curveballs.
2022		J. Hannaham Didn’t Nobody Give a Shit 247: It don’t seem like you could pull that kinda curveball on no PC.