noun
an idea or belief that is foolish or lacking in good sense
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The idea that money can buy happiness is a fool notion that has been debunked by numerous studies.
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Thinking that skipping class won't affect your grades is a fool notion, trust me.
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Believing that you can eat junk food all day and still stay healthy is a fool notion, dude.
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The belief that you can achieve success without hard work is a fool notion that will only lead to disappointment.