Rookie: noun a recently enlisted or new member, as of a military, police, or sports organization. The word rookie usually implies an inexperienced, raw, untried individual. Compare newbie, greenie, cherry. I would have never given that guy the keys to the hovercraft if I'd realized he was just a rookie.

To rook someone is to trick them into doing something they would not wish to do. It was once a tradition in the United States military to explain to new enlists that they are "rookies" because Uncle Sam has "rooked" them into doing a very difficult and thankless job for low pay.

While it is possible that this usage may have influenced the word, rookie seems to be a humorous derivation of the word "recruit." It is still used by sports teams and police forces, the word appears to have fallen out of common military use sometime after WWII and is now considered a bit old-fashioned by soldiers.

The word first appeared in Rudyard Kipling's Barrack-Room Ballads,, from 1892. By the first World War, it was in use by both military and sports groups.

Wikipedia offers us a theory (I know! I know! Just bear with me for the sake of completeness) that the word rookie is somehow tied to the rook in chess, as it is frequently the last piece to be developed in a game, like a late-appearing newbie. There is really no evidence to support this theory, however.

The word may also be used as an adjective—"My rookie year on the force was really tough." This is not as common as the noun form.

Room, Adrian, the Fascinating Origin of Everyday Words, (NTC Publishing Group, Chicago, 1986).
Online etymology dictionary:
ESPN Magazine on line, the answer guy:

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