The lyric is “Britannia, rule the waves.” The line comes from “Rule, Britannia!”, a British patriotic song written by James Thomson and set to music by Thomas Arne in 1740.
The song first appeared as the finale of Alfred, a masque about Alfred the Great, performed at Cliveden for Frederick, Prince of Wales. Its imagery links Britain’s liberty and security to maritime power, making “waves” the natural and canonical completion.
A common mix-up is “Britannia rules the waves,” with a terminal “s.” That version became popular in the Victorian era, when Britain’s naval dominance made the statement sound descriptive rather than exhortatory. The original wording is an imperative: Britannia is being urged to rule.