WordReference Random House Learner's Dictionary of American English © 2017
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.

WordReference Random House Unabridged Dictionary of American English © 2017
doubt•ful  (doutfəl),USA pronunciation adj. 
  1. of uncertain outcome or result.
  2. admitting of or causing doubt;
  3. unsettled in opinion or belief;
  4. of equivocal or questionable character:His tactics are highly doubtful.
doubtful•ly, adv. 
doubtful•ness, n. 
  • Middle English douteful. See doubt, -ful 1350–1400
    • 1.See corresponding entry in Unabridged undetermined, unsettled, indecisive, dubious, problematic.
    • 2.See corresponding entry in Unabridged unsure, indeterminate.
    • 3.See corresponding entry in Unabridged irresolute, vacillating, hesitant.
      Doubtful, dubious, incredulous, skeptical imply reluctance or unwillingness to be convinced. To be
      doubtful about something is to feel that it is open to question or that more evidence is needed to prove it:to be doubtful about the statements of witnesses.Dubious implies vacillation, unsureness, or suspicion:dubious about suggested methods of manufacture.Incredulous means unwilling or reluctant to believe:incredulous at the good news.Skeptical implies a general disposition to doubt or question:skeptical of human progress.
    • 4.See corresponding entry in Unabridged shady.
    • 1, 2.See corresponding entry in Unabridged certain.
    See  doubt. 

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

'doubtful' also found in these entries:

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