In general, someone or something having the qualities of a genius and an ignoramus. Specifically, it may mean one of the following:
1) Someone who is very intelligent but who lacks tact and common sense. Someone who is book-smart but who has poor social skills.
2) An intellectual person who lacks the communication skills to articulate his or her knowledge in a sophisticated or even coherent manner.
3) Someone who sometimes seems very smart, and at other times very stupid.
4) A person with great intellectual potential who squanders his or her mental gift for the sake of popularity or convenience.
5) An intelligent person who uses his or her gift inappropriately. Someone who is smart, but not smart enough to stay out of trouble, or who uses his or her gift to get into trouble. For example, a dog who knows how to open doors or drawers to get at food, or knows how to root through a trash can without spilling it.
My dog knows enough to open the cupboard door, slide open the lid to the bread box, and open the bag of bread without knocking anything onto the floor, but he's not smart enough to stay out of trouble in the first place. He's such an genioramus!