Synonym: dubious, dubitable, in question, tentative. Similar words: doubt, no doubt, without doubt, undoubtedly, fretful, double, frightful, deceitful. Meaning: ['daʊtfʊl] 
adj. 1. open to doubt or suspicion 2. fraught with uncertainty or doubt 3. unsettled in mind or opinion.
adj. 1. open to doubt or suspicion 2. fraught with uncertainty or doubt 3. unsettled in mind or opinion. 
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(31) We were doubtful about the practicability of the plan.
(32) He yawned a doubtful relay.
(33) The arrangement is of doubtful legality.
(34) We were doubtful about the product's usefulness from the start.
(35) It was doubtful that the money would ever be found again.
(36) It is doubtful that the missing airmen will ever be found.
(37) It is doubtful whether Tweed, even with his fluent French, passed for one of the locals.
(38) It is doubtful whether it will be possible to carry through the education reforms as quickly as the government hopes.
(39) Even if we could go, which is highly doubtful[Sentencedict.com], John wouldn't be able to come with us.
(40) It was doubtful whether the patient would survive the operation.
(41) It is doubtful whether this research is of any benefit to mankind.
(42) 'Everything's going to be all right, you'll see.' Jenny looked doubtful .
(43) I was still very doubtful about the chances for success.
(44) Let's try to clear up our difficult and doubtful points.
(45) The poems are supposed to be by Sappho, but they are actually of doubtful authenticity.
(46) It was doubtful whether the Conservatives could chalk up a fourth successive election victory,[www.Sentencedict.com] but they did.
(47) With her injuries it's doubtful that she'll ever walk again.
(48) Everyone was doubtful at first, but his statement in detail held up.
(49) I am doubtful whether I can solve the puzzle, but I'll have a shot at it.
(50) It is doubtful whether our resources will hand out for more than two or three days more.
(51) It is doubtful whether the government will ever accede to the nationalists' demands for independence.
(52) She was rather doubtful about the wisdom of eating the two-day-old food.
(53) They paused at the top of the stairs, doubtful as to which way to go next.
(54) In doubtful and borderline cases teachers will take the final decision, based on the student's previous work.
(55) The teacher is doubtful about having parents working as classroom assistants.
(56) Giggs is injured and is doubtful for the game tomorrow .
(57) At the beginning of the war things were looking very doubtful.
(58) It is doubtful whether/if they ever reached the summit before they died.
(59) Some British banks have used doubtful property revaluations to improve their capital ratios.
(60) It's doubtful that we'll finish this tonight.
More similar words: doubt, no doubt, without doubt, undoubtedly, fretful, double, frightful, deceitful, double up, delightful, at full speed, double back, double-dealing, double jeopardy, subtle, subterfuge, trouble, troubled, dour, do up, troublesome, hand out, dough, hold out, weed out, washed out, in and out, find out, stand out, send out.