noun an airman in the Royal Air Force. In army and navy use. From the colour and consistency of a blue ointment used to treat CRABS (pubic lice); the blue is of a similar shade to the Royal Air Force uniform. However, this derivation may not be direct, nor strictly accurate. From the early C20, Admiralty grey paint was called 'crab-fat' and the anti-lice ointment was claimed as the inspiration for that shade. It seems equally likely, therefore, that this later use should derive from the grey paint UK, 1961