People who are members of ARRSE, obviously. They are easily identified by their look of exasperated resignation and dress sense seemingly at odds with the rest of civilised society. Usually outraged and often Daily Mail readers, their blood pressure is invariably in the high 200/150+ range. They are institutionally cynical and occasionally dangerous. Best avoided.
Also a slang term for the good folk who hang out near Hampstead Heath in the early hours of the morning.