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):
amateurishly adv 1: in an amateurish manner; "he performed the piece amateurishly" [ant: expertly, like an expert]