The Collaborative International Dictionary of English v.0.48:

Greenhorn \Green"horn`\, n.
   A raw, inexperienced person; one easily imposed upon. --W.
   Irving.
   [1913 Webster]

WordNet (r) 3.0 (2006):

greenhorn
    n 1: an awkward and inexperienced youth [syn: cub,
         greenhorn, rookie]