stupid and silly behaviour

stupidity

stu·pid·ity [stupidity stupidities]   [stjuːˈpɪdəti]    [stuːˈpɪdəti]  noun (pl. stu·pid·ities)
1. uncountable, countable, usually plural behaviour that shows a lack of thought or good judgement
• I couldn't believe my own stupidity.
• the errors and stupidities of youth

• The police were astonished at the sheer stupidity of drivers in going so fast in the fog.

2. uncountable the state or quality of being slow to learn and not clever or intelligent
• He faked stupidity to try to escape punishment for the crime.  
Example Bank:
• Her answer showed the most incredible stupidity.
• I can't believe the stupidity of some people.
• I now realize the stupidity of that statement.
• It was sheer stupidity on the part of the crew.
• It's hard to know if it's corruption or just plain stupidity by the authorities.
• Through his own stupidity, he missed an excellent opportunity.
• I couldn't believe my own stupidity.

• The police were astonished at the sheer stupidity of drivers going so fast in the fog.