Senior-most members of the royal family are expected to assist the British military in some capacity, and Prince William became a British Army officer in 2006. He went on to earn even more titles over the years, and the prince's military friends also came up with an interesting nickname for him.
When Prince William joined the military, he had to assume a surname instead of going by his royal moniker, so he chose "Wales," the surname he and his brother Harry were given during their time in school. "William Wale's" friends came up with the nickname "Billy the Fish,” referring to a character from the British comic "Viz."
Even William's teachers in the Royal Air Force began to use this nickname, and William wore it as a badge of honor. The prince is still friends with his military buddies today, and they even came up with a joke name badge reading "Billy the Fish, that was presented to William when he finished his pilot training course in 2008.