Meaning of ninety-day wonder

nine'ty-day won'der

Pronunciation: (nīn'tē-dā"), [key]
— Informal. Informal.
  1. an officer commissioned in a branch of the armed forces after an unusually short training period, esp. after a three-months officers' training course during World War II.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.