New Oxford American Dictionary rookie | ˈrʊki | noun informal a new recruit, especially in the army or police: [as modifier] : a rookie cop. • a member of an athletic team in his or her first full season in that sport. ORIGIN late 19th century: perhaps an alteration of recruit, influenced by rook.

A beginner who is likely to make mistakes. Also called a "probie" or "newbie" or "noob" (very slang).

Someone who is new to the job (or club or team) is a rookie. For US sports, specifically, a "rookie" is someone who is in their first year of professional sports.

a rookie is a beginner. someone who lacks experience.