To make someone realize that something they believed in or hoped for is not as good or real as they thought.
From French 'désillusion,' formed by the prefix 'dis-' (opposite) plus 'illusion' (false belief). It emerged in the 18th century when people wanted a word for losing false hopes.
Disillusionment is actually a sign of growing up—it means you've gained enough experience to see through something that fooled you before. The word itself arrived late to English because we needed to describe a very modern feeling!
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