The phrase "a foolishness" is correct and usable in written English.
It can be used when referring to a specific instance or type of foolish behavior or action.
Example: "His decision to invest in that failing company was a foolishness that cost him dearly."
Alternatives: "an act of foolishness" or "a moment of folly".
That's a foolishness all its own.
It was a foolishness to think it had ever gone away.
The statue became the subject of attention in pulpits and in religious publications, the claims made for it fulminated against as a foolishness.
Recommending that the United States resume waterboarding and other torture techniques is not a sign of strength, it's a sign a foolishness.
In a time when art has to justify itself and when craft in prose is overlooked or seen as a foolishness – I like to stare at the undeniable power behind it all – the huge amoral force of story.
The chaplain of the Hialeah Police Department told the city council that Santeria was a sin, "foolishness," "an abomination to the Lord," and the worship of "demons". He advised the city council that "We need to be helping people and sharing with them the truth that is found in Jesus Christ". He concluded: "I would exhort you..
It was a time when a little foolishness was still all right.
He remembers thinking that it was an "act of foolishness, a horrible mistake". His wife was a thoughtful and at times energetic citizen, who organized small-scale campaigns for local issues; like him, she was opposed to Russia's involvement in the war in eastern Ukraine, but she was not a committed activist, let alone some sort of spy.
It's the "No puppet!" maneuver, which he deployed during a debate with Hillary Clinton, simply insisting upon things with an obstinate foolishness and waiting for a kernel of normality in his opponent to emerge.
He remembers thinking that it was an "act of foolishness, a horrible mistake".
