In essence it should communicate that someone is mostly preoccupied with his field of study and does not care much about other things going around him. He is an expert in his field but is not interested in "mundane" things. This leads to e.g.:
- Having messy, wild hair.
- Not caring how old the clothes are that he wears.
- Not interested in fulfilling other social norms.
The word does not have to include all the points but only the meaning put in bold letters, the rest kind of follows from that. It does not imply any madness or megalomania. Also the connotation should not be negative.
Example usage with adjective: Professor Smith is really ____ , but he is one of the best professors I know because he can engage students in a way that other lecturers cannot.
Example usage with noun: Professor Smith is a real ____ , but he is one of the best professors I know because he can engage students in a way that other lecturers cannot.