• unjullible,
• ungulible,
• ungullable,
• uungullible

• gullible,
• inescapable,
• incorrigible,
• impregnable,
• vulnerable,
• indelible,
• intelligible,
• irreversible,
• unbreakable,
• malleable,
• invulnerable,
• tangible,
• inevitable,
• formidable.

The adjective form of the word "ungullible" is "ungullible".

The plural form of "ungullible" is "ungullibles".

The singular form of the word "ungullible" is "ungullible". The word does not change in form when referring to a single person or thing.