The Collaborative International Dictionary of English v.0.48:

Amateurish \Am`a*teur"ish\, a.
   In the style of an amateur; superficial or defective like the
   work of an amateur. -- Am`a*teur"ish*ly, adv. --
   Am`a*teur"ish*ness, n.
   [1913 Webster]

WordNet (r) 3.0 (2006):

amateurish
    adj 1: lacking professional skill or expertise; "a very
           amateurish job"; "inexpert but conscientious efforts";
           "an unskilled painting" [syn: amateurish, amateur,
           inexpert, unskilled]

Moby Thesaurus II by Grady Ward, 1.0:

40 Moby Thesaurus words for "amateurish":
   amateur, clumsy, crude, dabbling, defective, deficient, dilettante,
   dilettantish, faulty, flawed, green, half-assed, half-baked,
   half-cocked, immature, jackleg, raw, sciolistic, semiskilled,
   shallow, smattering, sophomoric, superficial, unaccomplished,
   unbusinesslike, uncoached, unendowed, unfinished, ungifted,
   uninitiated, unpolished, unprepared, unprimed, unprofessional,
   unschooled, unskilled, untalented, untaught, untrained,
   untutored