1. In a doubtful manner; dubiously.

Nuttall's Standard dictionary of the English language By Nuttall, P.Austin

• doutfull,
• dutfully,
• doubtfull,
• doubtfuly,
• doubfull,
• duitfully,
• soubtfully,
• xoubtfully,
• coubtfully,
• foubtfully,
• roubtfully,
• eoubtfully,
• doubtfilly,
• doubtfullly,
• dountfuly,
• dountfilly,
• doutfuly,
• doubtfulyll,
• doubfulley,
• doubtfullay

• doubtful,
• doubtingly,
• dutifully,
• doubtfulness.

The adjective form of "doubtfully" is "doubtful".

The plural form of the word "doubtfully" is "doubtfullys". However, it is worth noting that "doubtfullys" is not a commonly used term.

The singular form of the word "doubtfully" is "doubtful"