To remove the class status or classification from something; to make something no longer fall into a particular class or category.
From dis- (reversal) + classify (from class + -ify, suffix meaning 'to make'). This is a theoretical back-formation following standard English morphology, though 'declassify' is the more established term for removing security classifications.
The prefix puzzle: why do we say 'declassify' for removing secret status but theoretically could say 'disclassify'? It's because 'de-' entered English first for this concept, and once a word takes root, competing formations rarely develop.
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