1. Not doubtful; plain; evident.

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

• yndoubtful,
• hndoubtful,
• jndoubtful,
• indoubtful,
• 8ndoubtful,
• 7ndoubtful,
• ubdoubtful,
• umdoubtful,
• ujdoubtful,
• uhdoubtful,
• unsoubtful,
• unxoubtful,
• uncoubtful,
• unfoubtful,
• unroubtful,
• uneoubtful,
• undiubtful,
• undkubtful,
• undlubtful,
• undpubtful

• convincing,
• incontrovertible,
• unfathomable,
• certain,
• irrefutable,
• unquestionable,
• undeniable,
• indubitable,
• doubtless.

The adjective form of the word "undoubtful" is "undoubtful" itself.

The plural form of "undoubtful" is "undoubtfuls".

The singular form of "Undoubtful" is "Undoubtful". The word does not have a distinct singular form as it is used the same way in both singular and plural contexts.