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 Quiz → IELTS Quiz → ← doubt dour → Study tips featuring "doubtful" Audit Your Own Writing to Find Gaps TOEFL Academic Vocabulary: Study by Topic Boost Your IELTS Writing With Precise Vocabulary Related words TOEFL swiftness GRE abash GRE abate GRE abbreviate GRE abbreviation SAT abdicate GRE abduct SAT abet GRE abhor GRE abide SAT abject TOEFL ablaze