ninety-day wonder

[nahyn-tee-dey]
noun Informal.
  1. an officer commissioned in a branch of the armed forces after an unusually short training period, especially after a three-months officers' training course during World War II.

Origin of ninety-day wonder

An Americanism dating back to 1915–20
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018