Why are raf called crab fats?

The reason the RAF are called crab fats by the Royal Navy goes back a few years and lies in the fact that naval personnel used to use the grease that the gun shells were caked in to get rid of crabs which they had picked up after visiting some brothel overseas.

The term crab fat derived from the grease they used, it happened to be the same colour as the RAF uniform. Hence the term.