Idealism is not a slur, but the philosophy that liberalism is built on. It posits that reality is shaped by people's ideas in their brains.
This leads to all sorts of horrifying stuff:
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People try to make their lies reality by trying to force other people to believe them.
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People dehumanize people who disagree with them since only human's ideas are the ones that supposedly shape reality.
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Whenever the idealists' ideas don't become reality, instead of questioning their ideology, they can just blame everyone who disagreed with them, for "bending reality the other way".