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" Map Each Word Into a Connected Network Audit Your Own Writing for Lexical Resource Track How TOEFL Words Behave With Others 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