1. the quality or state of being uncertain or not confident about something
2. the quality of being questionable or open to doubt; lack of clarity or reliability

Examples

• The doubtfulness in her voice made it clear she wasn't fully convinced.
• There is some doubtfulness about whether the plan will actually work.
• His doubtfulness regarding the project's success was evident from his hesitant response.
• The doubtfulness of the evidence made the jury reluctant to convict.
• She expressed doubtfulness about the company's financial stability.
• The doubtfulness of his claims led many to question his credibility.