At one point in history a "0day" referred to a pirated release of software on/before it hit the shelves (and/or the crack that was released simultaneous to protected software aka "warez").

It was years later that a "0day" went from a copy protection removal/crack ("0day warez") to its more general modern usage in computer security.

See: https://www.google.com/books/edition/_/8ETRQhDytIsC?hl=en&gb...