The letters relate to the Queen and stand for the Latin Fidei Defensatrix, or ‘Defender of the Faith’. Fidei Defensor (the male version) was a title granted to Henry VIII by the Pope in 1521, after the King criticised the Protestant reformer, Martin Luther.
The title was revoked when Henry broke with Rome in 1530, but in 1544 the English Parliament conferred it on the King who, as supreme governor of the Church of England, was defender of the Anglican faith.
‘Fid Def’ or the letters ‘FD’ have appeared on coins since the 18th century. It was left off the new florin or two-shilling piece in 1849, and the coin became known as the ‘godless florin’ – it was rapidly redesigned.