Curveball was the designation for a claimed "Iraqi chemical engineer" who the United States claimed had served as an informant. Curveball would be the attributed source of pivotal information concerning weapons of mass destruction leading up to the 2003 Invasion of Iraq. The name Curveball is a reference to a curveball baseball pitch, which is US English slang for something that behaves indirectly, erratically, or surprisingly. Wikipedia
The definitive Wikipedia entry for Curveball. Wikipedia is the biggest multilingual free-content encyclopedia on the Internet. en.wikipedia.org/wiki/Curveball_%28informant%29