Fluctuat nec mergitur (or FLVCTVAT NEC MERGITVR when an inscription written in capitals) is a Latin phrase meaning "She is tossed by the waves, but is not sunk":
This phrase is the motto of Paris, France, and is present in the city coat of arms depicting a ship floating on a rough sea. Both motto and city arms have their origins in the Seine River boatsman's corporation; this powerful hanse ruled the city's trade and commerce as early as the Roman era. Although this corporation through the centuries became an entity resembling more a municipal government than a trade organization, they maintained their original arms and motto, and it is for this that the Mairie de Paris bears them still today.
The phrase is also the motto of Albert Messiah's classic textbook on quantum mechanics.