Definition of doubtful adjective from the Oxford Advanced Learner's Dictionary

     

    doubtful

     adjective
    adjective
    BrE BrE//ˈdaʊtfl//
     
    ; NAmE NAmE//ˈdaʊtfl//
     
     
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  1. 1(of a person) not sure; uncertain and feeling doubt synonym dubious doubtful (about something) Rose was doubtful about the whole idea. doubtful (about doing something) He was doubtful about accepting extra work.
  2. 2unlikely; not probable doubtful (if…) It's doubtful if this painting is a Picasso. doubtful (that…) With her injuries it's doubtful that she'll ever walk again. doubtful (whether…) It's doubtful whether the car will last another year. doubtful (for something) He is injured and is doubtful for the game tomorrow (= unlikely to play).
  3. 3[not usually before noun] (of a thing) uncertain and likely to get worse At the beginning of the war things were looking very doubtful.
  4. 4[only before noun] of low value; probably not genuine or of a quality that you can rely on synonym dubious This wine is of doubtful quality.
  5. Extra examples Even if we could go, which is highly doubtful, John wouldn’t be able to come with us. She was rather doubtful about the wisdom of eating the seafood. She was rather doubtful about the wisdom of eating the two-day-old food. A doubtful look crossed her face. It is doubtful if this painting is a Picasso. It is doubtful that his words will change the situation much. It is doubtful whether the car will last another year. Terry is injured and is doubtful for the game tomorrow. With her injuries it’s doubtful that she’ll ever walk again.
See the Oxford Advanced American Dictionary entry: doubtful