The Collaborative International Dictionary of English v.0.48:

Unprepossessed \Unprepossessed\
   See prepossessed.

Moby Thesaurus II by Grady Ward, 1.0:

19 Moby Thesaurus words for "unprepossessed":
   detached, disinterested, dispassionate, equitable, evenhanded,
   fair, impartial, impersonal, indifferent, just, neutral, objective,
   unbiased, uncolored, undazzled, uninfluenced, unjaundiced,
   unprejudiced, unswayed