The Collaborative International Dictionary of English v.0.48:

Unprofessional \Unprofessional\
   See professional.

WordNet (r) 3.0 (2006):

unprofessional
    adj 1: not characteristic of or befitting a profession or one
           engaged in a profession; "described in unprofessional
           language so that high school students could understand
           it"; "was censured for unprofessional conduct";
           "unprofessional repairs" [ant: professional]

Moby Thesaurus II by Grady Ward, 1.0:

46 Moby Thesaurus words for "unprofessional":
   amateur, amateurish, everyday, improper, incompetent, inefficient,
   inexperienced, inexpert, inferior, lax, lay, low-quality,
   negligent, ordinary, plain English, poor, second-rate, semiskilled,
   shoddy, sloppy, unaccomplished, unbecoming, unbefitting,
   unbusinesslike, uncoached, understandable, undignified, unendowed,
   unethical, unfinished, unfitting, ungifted, uninitiated,
   unpolished, unprepared, unprimed, unprincipled, unscholarly,
   unschooled, unseemly, unskilled, untalented, untaught, untrained,
   untutored, unworthy