Definition
Of a nation or society, to be deprived of educated or talented people, who in turn migrate to larger cities or more developed nations.
To become mentally or psychologically fatigued or exhausted.
Origins
Coined by spokesmen for the Royal Society of London in the early 1950s to describe the outflow of scientists and technologists to the United States and Canada.
How People Actually Use It
When you study way to hard and your brain is drained.
""Dude, I had 3 exams to study for over the long weekend and I pushed off all my studying till 10:30 last night" "Woahh, you must have major brain drain!!" "heck yes""
Source: Urban Dictionary