Doubtful means uncertain or unsure about something. When you're doubtful about whether it will rain tomorrow, you're not convinced either way. When a friend promises to finish their part of a group project on time but has missed deadlines before, you might feel doubtful about whether they'll really come through.

The word also describes something that seems unlikely or questionable. If someone claims they can run a mile in three minutes, that seems doubtful because it would break the world record. A doubtful excuse is one that sounds suspicious or hard to believe.

You can see how these meanings connect: when you're doubtful about something, you're questioning whether it's true or will happen. Your doubts come from having reasons to be uncertain. A scientist might be doubtful about a new theory until she sees solid evidence. A coach might be doubtful that an injured player can compete this weekend.

Being doubtful isn't the same as being negative. Sometimes doubt is useful, helping you think carefully before believing everything you hear or making risky decisions. When something sounds too good to be true, doubt encourages you to investigate further.