Stupidity is behaviour that shows a serious lack of sense. Not ignorance, which can be fixed by learning.
The word describes acts more comfortably than people, which is useful, since calling a decision stupidity is a criticism you can make without insulting anybody directly. It is a mass noun. There is a well-known line about never attributing to malice what can be explained by stupidity, which is worth remembering in most disputes. The adjective stupid is far commoner and much blunter.
1nounstupidityBehaviour showing a serious lack of intelligence or sense.
“The accident was caused by sheer stupidity.”
Latin
stupiditātem
English, today
stupidity
From Latin stupiditātem, accusative of Latin stupiditās, equivalent to stupid + -ity.
Etymology from Wiktionary, CC BY-SA