To cause someone to lose their illusions or false beliefs; primarily a British spelling variant.
From disillusion + -ise (British spelling suffix for verbs, from Old French -iser). The British '-ise' ending comes from Latin '-izare,' distinguished from American '-ize' spellings since the 19th century.
British and American English took different paths with this verb—the British 'disillusionise' versus American 'disillusion' or 'disillusionment' shows how spelling conventions diverged in the 1800s when the two countries' publishing industries standardized differently.
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