Rookie \ROOK-ee\ , noun;
1. An athlete playing his or her first season as a member of a professional sports team
2. A raw recruit, as in the army or on a police force
3. A novice; tyro
Rookie is first attested in 1868 and it's origin is uncertain, though of course there are some theories. It may be a shortening of recruit with the -y suffix added. However, the change from -e- to -oo- (/ə/ to /ʊ/, if you know IPA) is hard to explain without some outside influence. In this case, rook ("to cheat, swindle") or the first part of rookery (Military Slang "the part of the barracks occupied by subalterns") may have provided that influence. It could also be that recruit has nothing to do with it and rookie comes directly from either rook given above (plus -y).
The -y suffix is used to form pet names and diminutives
So, I've never heard of tyro before, but it turns out it is basically rookie's predecessor in English. It dates to 1611 from Latin tiro and means "a beginner or learner in anything; a novice."