To remove the classified status from secret government documents; to declassify.
From de- (reverse/remove) + class (classification level). A shortening of declassify, commonly used in government and military contexts.
The US government's classification system creates so much bureaucratic language that even the action of releasing secrets got shortened to 'declass' in internal documents—it's the kind of jargon-bred shortening that sounds awkward to outsiders but feels natural to people who use it daily.
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