Someone who is a bit of an idiot on occasion. They can be intellegent most of the time, but once in a blue moon the will do something so clueless that you can't understand how they ever got that job or into that school.
"Crystal got 2400 on the SAT's, but she still has no idea that fish need water to breathe..."
"Damn, she's a doobiefrank, ain't she?"
"Yea man, your telling me."