Causing someone to realize that something they believed in is false or not as good as they thought.
From disillusion (to remove illusions, from dis- + illusion) + -ing (present participle or adjective suffix). The word entered English in the 18th century as the verb form became common.
The 20th century loved this word—'disillusioning' captures the specific disappointment of discovering reality doesn't match your hopes, and it became especially popular after World War I when an entire generation needed a word for their shattered ideals.
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