1. feeling uncertain about something; not sure or confident
2. unlikely to be true, successful, or reliable; questionable

Examples

• I’m doubtful that we’ll finish the project on time.
• She gave him a doubtful look when he made his excuse.
• The weather forecast is doubtful for tomorrow’s picnic.
• It’s doubtful whether the old bridge can support heavy traffic.
• The team’s chances of winning are doubtful after losing their best player.
• He was doubtful about investing in the risky venture.
• The authenticity of the painting is doubtful.