"raw recruit," 1892 in that spelling, popularized by Kipling's "Barrack-Room Ballads," of uncertain origin, perhaps from recruit, influenced by rook (n.1) in its secondary sense, suggesting "easy to cheat." Barrère ["A Dictionary of Slang, Jargon & Cant," 1890] has "Rookey (army), a recruit; from the black coat some of them wear," so perhaps directly from rook (n.1). Came into general use in American English during the Spanish-American War.
The rapid growth of a word from a single seed transplanted in a congenial soil is one of the curiosities of literature. Take a single instance. A few weeks ago there was not one American soldier in a thousand who knew there was such a word as "rookey." To-day there are few soldiers and ex-soldiers who have not substituted it for "raw recruit." ["The Midland Monthly," December 1898]