TOEFL IELTS doubtful Definition Not certain; unlikely or questionable. Example "It's doubtful that we'll finish the project by Friday." Sign in to save TOEFL PDF — $25 IELTS PDF — $25 ← doubt dour → Study tips featuring "doubtful" TOEFL Academic Vocabulary: Study by Topic Boost Your IELTS Writing With Precise Vocabulary Related words TOEFL brighten GRE abash GRE abate GRE abbreviate GRE abbreviation SAT abdicate GRE abduct SAT abet GRE abhor GRE abide SAT abject TOEFL ablaze