A young, malicious hacker who isn't smart enough or skilled enough to create custom hacking software, so must rely on programs created by other people. Also: kidiot.
"Rarely found in IT-related jobs, 'kiddiots' download hacking tools and run basic attacks to gain credence among their peers."
Paul Allen, "Information culture is in need of development," Network News, September 19, 2001
Microsoft is also painfully aware of the consequences of public disclosure of exploits as thousands of script kidiots attack vulnerable systems.
—"Unix Security," Carole Fennelly, SunWorld, October 1, 2000
Today's term a blend of kiddie and idiot is synonymous with the older phrase script kiddie. In fact, kiddiot seems to have arisen as a shortened form of the phrase script kiddiot, although the latter is more often seen as script kidiot, as the earliest citation demonstrates.
For the record, here's an even earlier usage, although the sense is completely different:
Maybe people have given up trying to keep companies from exploiting kids.
After all, long ago, Saturday morning 'kidiot' cartoon shows became open season on tiny (future) consumers.
Ernest Tucker, "Song Deal Is Juicy for McDonald's," Chicago Sun-Times, July 6, 1993