Opening something by pulling down or apart a zipper, or revealing something hidden or private.
From 'un-' (reversal prefix) plus 'zip,' which was a trademark name that became generic. 'Zip' itself is imitative, matching the sound a zipper makes.
The zipper is a surprisingly modern invention from 1913—before that, people used buttons and hooks, which is why old clothes took forever to put on and remove!
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