1. the quality or state of being stupid; lack of intelligence or common sense
Usage: less common than ‘stupidity’

Examples

• The stupidness of his decision became clear when he lost all his money.
• She couldn’t believe the stupidness of the mistake on the test.
• His stupidness in handling the situation made things worse.
• The stupidness of the plan was obvious to everyone except him.
• They laughed at the stupidness of the comedy show.
• The stupidness of the argument frustrated the teacher.