Althenick, Beefy and Ken.
At my mandatory Service history session at PEMBROKE, many, many years ago, we were told it was to do with the drums of crab fat that followed the RNAS around. The story was that it was used as a preservative on early flying machines. Accordingly, in the age-old tradition of banter, aviators and their riggers/mechanics/fitters were "Crab Fats". When the RNAS was subsumed within the RAF, the nickname went with it. That would predate any blue grey uniform, though.
Anorak dried and aired.