Feeling uncertain or lacking conviction about something. adjective
She was doubtful about the success of the new project.
He gave a doubtful look when asked if he could finish the task on time.
Not likely to be true or to happen; uncertain. adjective
It's doubtful that the team will win the championship this year.
The weather forecast is doubtful, so we might need to change our plans.
Of questionable quality or reliability. adjective
The authenticity of the painting is doubtful.
The source of the information is doubtful, so we should verify it.