In A Nutshell
During World War I, members of the British intelligence service MI6 used, um, “bodily fluids” as invisible ink to pass secret messages. This was discovered by a man hilariously and ironically named Mansfield Cumming.
The Whole Bushel
When the British were fighting in World War I, the nation’s spies working for the intelligence organization MI6 were looking for new ways to pass classified information. They soon discovered that a man’s semen worked very well as invisible ink.
The idea to use semen as invisible ink came from a spy named Mansfield Cumming, and yes, that was his actual name. Code-named “C” in the diary of intelligence officer Walter Kirke, Cumming proposed using this certain bodily fluid to write secret messages as it could not be revealed with the use of iodine vapor.
This method never really caught on, presumably because of the awkward way in which this invisible ink needed to be procured, as well as the fact that the idea of writing with semen probably led to more spies reacting with uproarious laughter than taking the method at all seriously.
Show Me The Proof
MI6 ‘used bodily fluids as invisible ink’
Brit Spies Used Semen as Invisible Ink