Webster's English Dictionary

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doubtful

doubt.ful \'dau.t-f*l\ \-f*-le-\ aj 1: marked by uncertainty or lack of 
   clarity  2a: lacking a definite opinion, conviction, or determination  2b1: 
   uncertain in outcome : UNDECIDED  2b2: not predictable  3: marked by 
   qualities that raise doubts about worth, honesty, or validity  and DUBIOUS 
   indicate uncertainty in reference to persons or unpredictability in 
   reference to events and situations. DOUBTFUL implies simply a lack of 
   certainty or conviction; DUBIOUS stresses the unlikelihood of an ultimately 
   favorable outcome or judgment; PROBLEMATIC describes a situation having an 
   entirely unpredictable outcome; DOUBTFUL, applied as an estimation or 
   judgment, may question worth, honesty, or validity so strongly as to imply 
   their absence or insufficiency; DUBIOUS stresses usu. well-grounded 
   suspicion or mistrust; QUESTIONABLE may mean simply open to question but 
   often connotes falsity, unsoundness, or immorality that is strongly 
   suspected but may be asserted only in guarded statements or hints - 
   doubt.ful.ly av SYN syn DUBIOUS, PROBLEMATIC, QUESTIONABLE: DOUBTFUL