90 Day Wonder is a pejorative term for a military officer who earned his position through Officer Candidate School. The program was designed during World War II as a means of expanding the numbers of officiers primarily during wartime. Unlike members who go to West Point or other Officer academies, these men would become officers after only a short time (at one point as short as three-months, hence the name). The term has since also become a term of affection.
Another new phrase from David Fears’s Dark Blonde, a book that expanded my vocabulary more than I ever expected.