Why do the British Army and Royal Navy call the RAF 'crabs'?

According to this site - [no longer available] - this is the definition - Crabs: the RAF (when asked something, they shrug their shoulders and walk off sideways)

[The definition here was apparently from a British military humor magazine. The link is no longer active.]

Crab, crabfat - (RN) A member of the Royal Air Force. From the light blue color of the uniform, which is the same as that of the grease (known as ‘crabfat’) used on gun breeches, etc., in the RN. Accounts vary, but apparently the grease was called ‘crabfat’ because it resembled in color the ointment used to treat sailors for ‘crabs’ (pediculosis pubis, genital lice), and of which fat was a major constituent.
From [btinternet.com/~a.c.walton/navy/smn-faq/slang1.htm] link no longer exists