The letters "F.D" are inscribed on all British coins and stand for Fidei Defensor, which translates from Latin to "Defender of the Faith." Fidei Defensor has been used as a subtitle for all British monarchs and appears on the same side of the coin as the monarch's head.
The title of Fidei Defensor was originally bestowed by Pope Leo X to King Henry VIII of England in 1521. It has been used on coins, sometimes also as FID DEF, since the reign of King George I in 1714. The Royal Mint left the inscription off coins minted in 1849, but after an uproar of protest the initials were reinstated.