WordReference Random House Learner's Dictionary of American English © 2015
doubt•ful /ˈdaʊtfəl/USA pronunciation adj. 
  1. of uncertain outcome:a doubtful future.
  2. undecided in opinion or belief; hesitant:I'm doubtful about whether I made the right choice.
  3. of character or value that may not be genuine or real:The painting is of doubtful authenticity.
  4. unlikely; not probable:It's doubtful I'll finish the work on time.
doubt•ful•ly, adv.: We looked on doubtfully as he tried to lift the couch.

Collins Concise English Dictionary © HarperCollins Publishers::

doubtful /ˈdaʊtfʊl/ adj
  1. unlikely; improbable
  2. characterized by or causing doubt; uncertain: a doubtful answer
  3. unsettled; unresolved
  4. of questionable reputation or morality
  5. having reservations or misgivings

ˈdoubtfully adv ˈdoubtfulness n USAGE
It was formerly considered correct to use whether after doubtful (it is doubtful whether he will come), but now if and that are also acceptable

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