Dubious
Part of speech: adjective
Definition: Arousing doubt; questionable; open to suspicion. In disbelief; wavering, uncertain, or hesitating in opinion; inclined to doubt; undecided.
Example sentence: The War on Drugs employs millions - politicians, bureaucrats, policemen, and now the military - that probably couldn't find a place for their dubious talents in a free market, unless they were to sell pencils from a tin cup on street corners.
Doubtful
Part of speech: adjective
Definition: Subject to, or causing doubtExperiencing or showing doubtUndecided or of uncertain outcomeImprobable or unlikelySuspicious, or of dubious character
Example sentence: I was a little doubtful about the propriety of going to the Mammoth Cave without a gentleman escort, but if two ladies travel alone they must have the courage of men.