British spelling: a person or thing that causes disorganization or breaks down orderly systems; one who disorganizes.
From disorganise (verb) + -r (British agent suffix). The British -r ending corresponds to American -r in -izer words.
Calling someone a 'disorganiser' is quite rare—we usually say 'someone who's disorganized' rather than 'a disorganizer,' showing that agency nouns don't work for all verbs.
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