Cruel and unusual a poor standard?

In America, the justice system is forbidden to use 'cruel and unusual' punishment. All well and good, on the surface. But it seems to me that there are some major problems with this standard. If a punishment isn't cruel, then how can it be effective? Prison is certainly cruel, it is a type of psychological torture that you would be hard pressed to top for it's long term damage. And unusual is just a statement of frequency. If being slapped with a frozen halibut was decreed as the standard penalty for theft, then it would quickly stop being 'unusual'. It would, by definition, be the 'usual' punishment. Now, to be unconstitutional, a punishment most be both. But, since punishment is inherently cruel, and any sentence mandated by law would be 'the usual', no punishment on the books could be logically unconstitutional.

Let's change the standard to someting like 'malicious and weird'. Or we could realize that it's all subjecive and just say 'inappropriate'.

Thoughts?
The problem with our legal system is that we've let the Congresses and the lawyers screw it up. To stick somebody in prison actually punishes the taxpayers just as much because they have to pay to feed the criminal. We could save a fortune if we'd change to a purely restitution system where the penalty was equal to the crime plus legal fees. It doesn't make any sense to throw a thief in jail when the proper penalty would be that the thief has to pay the money back plus interest and legal fees and if said thief is unable to pay, he/she is enslaved to the victim until repayed.
Cruel and unusual may not be very specific. I think by cruel, they mean concentrating on hurting the criminal, as opposed to being a deterrent to further crime. For example, prison is supposed to be a deterrent, whereas removing fingernails would seem to be just for imposing pain.


Answer:
You raise a good point in that punishment would meet the constitutional test if it was cruel, but a number of jurisdictions did it. That's not much protection from cruel treatment.
The term 'cruel and unusual' is an old term from when kings ruled. Taken in its proper context, it means equal and fair punishment for all. It is just a matter of debate whether one punishment is cruel or unusual. It all depends on your point of view. Arguing about it accomplishes nothing.