This is the 11263rd most frequent Polish word.
głupotą
"Foolishness" or "stupidity."
"Głupotą" here describes a quality or characteristic of the behavior, emphasizing its reckless or nonsensical nature.
Zachowywanie się w ten sposób jest głupotą.
Behaving in this way is foolishness.
In this sentence, "głupotą" functions to portray the act of opposing facts as foolish or nonsensical.
Sprzeciwianie się faktom przez głupotą nie prowadzi do niczego dobrego.
Opposing facts through foolishness doesn't lead to anything good.
The word "głupotą" is used here as an instrumental case form of "głupota", meaning 'nonsense', showing the character of the remarks.