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]

Moby Thesaurus II by Grady Ward, 1.0:

95 Moby Thesaurus words for "greenhorn":
   abecedarian, alphabetarian, apprentice, arriviste, articled clerk,
   babe, baby, beginner, boob, boot, catechumen, chump, cinch,
   credulous person, cull, dabbler, deb, debutant, dilettante,
   dull tool, dunce, dupe, easy mark, easy pickings, emigrant,
   entrant, fall guy, fish, fledgling, fool, freshman, gate-crasher,
   gobe-mouches, greener, greeny, gudgeon, gull, ignoramus,
   illiterate, illiterati, immigrant, incapable, incompetent,
   inductee, infant, initiate, innocent, intruder, know-nothing,
   leadpipe cinch, learner, lowbrow, mediocrity, middlebrow, monkey,
   neophyte, nestling, new arrival, new boy, newcomer, no conjuror,
   no scholar, novice, novitiate, novus homo, parvenu, patsy, pigeon,
   plaything, postulant, prize sap, probationer, probationist,
   puddinghead, pushover, raw recruit, recruit, rookie, sap, saphead,
   schlemiel, settler, sitting duck, squatter, stooge, stowaway,
   sucker, tenderfoot, toy, trainee, trusting soul, tyro,
   unintelligentsia, upstart, victim



U.S. Gazetteer Places (2000):

Greenhorn, CA -- U.S. Census Designated Place in California
   Population (2000):    146
   Housing Units (2000): 84
   Land area (2000):     6.721582 sq. miles (17.408817 sq. km)
   Water area (2000):    0.000000 sq. miles (0.000000 sq. km)
   Total area (2000):    6.721582 sq. miles (17.408817 sq. km)
   FIPS code:            31029
   Located within:       California (CA), FIPS 06
   Location:             39.905794 N, 120.748665 W
   ZIP Codes (1990):
   Note: some ZIP codes may be omitted esp. for suburbs.
   Headwords:
    Greenhorn, CA
    Greenhorn


U.S. Gazetteer Places (2000):

Greenhorn, OR -- U.S. city in Oregon
   Population (2000):    0
   Housing Units (2000): 7
   Land area (2000):     0.100683 sq. miles (0.260768 sq. km)
   Water area (2000):    0.000000 sq. miles (0.000000 sq. km)
   Total area (2000):    0.100683 sq. miles (0.260768 sq. km)
   FIPS code:            31050
   Located within:       Oregon (OR), FIPS 41
   Location:             44.708737 N, 118.496872 W
   ZIP Codes (1990):
   Note: some ZIP codes may be omitted esp. for suburbs.
   Headwords:
    Greenhorn, OR
    Greenhorn