'Who Shaves the Barber'

I hear this phrase from time to time and guess, from the context, that it raises the question as to who is the final arbitrator. However, I've really no idea of the origin and stand to be corrected on the meaning. Can anyone help?

The phrase derives from a paradox put forward by Bertrand Russell: "in a certain town there is a barber who shaves everybody who does not shave himself. So who shaves the barber?" It is clearly a question to which there is no possible answer.