|Ryder Cup rookies 2014|
The word is most commonly used in American sports commentary to describe a team player in their first season - a rookie NFL quarterback, example, or a new player on the Ryder Cup team. Using a rookie in a high-pressured situation carries a risk as you can never be certain how they will react.
There is also a suggestion that youth and/or inexperience makes a rookie more likely to make elementary mistakes.
Where does the word come from?
The origin is uncertain but the OED suggests that it may be a corruption of the word recruit. It's first citation is from Rudyard Kipling's Barrack-Room Ballads (1892)
So 'ark an' 'eed, you rookies, which is always grumblin' sore, referring to rookies in the sense of raw recruits to the British Army.