Bravo Zulu is really an open naval secret. The origins of Bravo Zulu (or BZ in short) date back to the 1940s.
Navy ships in the fleet used to pass messages by morse code and through flag signals. BZ is a combo of both the Bravo and Zulu nautical flags being flown together as a way to convey “Good job or well done” between ships.
Bravo Zulu has been adopted by mariners across the globe as a universal signal as this short video below demonstrates.