A person who thinks they know everything who really knows nothing at all. After two weeks of hanging around a dunno all, you know everything they know and that have not gathered one bit of intelligence when you catch up with them 4 years later. The stories and un-applied knowledge repeats itself.
Minion: have you seen Jo lately
Thinker: yEah, i havent seen her for years and she still has the same stories as back then. The only time she stopped talking was to listen for her echo!
Minion: Figures, I always thought she was a dunno-all!