means A beginner or first-year participant in a field, sport, or job — someone new enough to be expected to fumble.

from A late-19th-century word, most likely an alteration of 'recruit,' worn down and reshaped in casual speech (some also point to 'rook,' the cunning bird whose name became slang for cheating a novice). It surfaced in military slang and was popularized in American sports, where each new season minted a fresh crop of rookies.