It's funny in the context of the Monty Python sketch. XD Basically, a British man writes a joke so funny that anyone who reads/hears it dies lauging. As part of the war effort against Hitler, it's translated into "German" and read out loud by the Britsh troops on the battlefield when they face the German army. You can see the sketch here: [link]
Though, from the number of "deaths" on DA, it's probably better that I don't know. XD
Awesome stamp!