Stupidity (noun)

1. Lack of intelligence or common sense.
2. A foolish or senseless act.

Origin:

From the latin word "stuporus" meaning "dazzled, senseless".

Examples:

1. The stupidity of the decision made by the government was clear to everyone.
2. His constant mistakes are a clear indication of his stupidity.
3. It's a sheer stupidity to think that you can get away with cheating on the exam.
4. His stupidity cost him his job.
5. It was a act of stupidity that he did not wear the helmet while riding the bike.