Excessive bail shall not be required, nor excessive fines imposed, nor cruel and unusual punishments inflicted.
Sherry Colb has a column out entitled "Evaluating the Eighth Amendment's Ban on Only Cruel and Unusual Punishments." It deals with the "unusual" aspect, which often is not addressed when discussing the provision. [See, e.g., how little space is given to it here.] The clause, reasonably enough, is generally understood as a term of art, "cruel and unusual" read together to mean something. As Justice Brennan once noted: "The question, in any event, is [arguably] of minor significance; this Court has never attempted to explicate the meaning of the Clause simply by parsing its words."
Prof. Colb notes "unusual" is a way to limit the discretion that "cruelty" alone might allow -- if something is rare, it is understood to be suspect. But, she notes, rarity alone is not the test of cruelty. If something cruel is popular or there is some precedent, it is still cruel (understanding that this is a matter of degree); some disagreements aside, this should be true in the constitutional sense too. But, as she admits, the text only allows this be to a point. Also, perhaps the unusual nature is done because of "a humane purpose in selecting it," which again underlines the term has to be understood in context.
In his concurrence in Furman v. Georgia (death penalty) (linked above), Justice Marshall discussed the origins of the Cruel and Unusual Punishment Clause. He suggests the use of "unusual" might have been "inadvertent." The use of "illegal" punishments in one draft suggests a core aspect of the concern here -- the fear that an executive (king) would use unauthorized, that is in some sense uncommon (much law at the time judge made "common law") punishments. It is not assumed they would do this for the good of the criminals, but even if they allegedly did, it could very well be misguided if "unusual" punishments were used.
Thus, and people like John Hart Ely Jr. supported this view, "unusual" suggests that there is a type of due process aspect here, a way to prevent "arbitrary" punishments. Likewise, an equal protection component -- if someone is being treated in a way others are not, that is in an unusual way, it is problematic. As suggested above in both directions, this is not an absolute barrier. Just because everyone jumps off the cliff, we need not do it; and, just because no one does something, it doesn't make it wrong. Still, it is a sound general principle, particularly given our system of government and its basic principles.
So, in a constitutional sense, "unusual" can both mean "not common" and "unauthorized." Somewhat akin to Prof. Colb's thesis, the latter definition can be on some level idealistic, like "all men are created equal" holds up as a norm even if slavery is allowed. After all, "unusual" is not found in a vacuum, but in a Constitution with many other provisions. Finally, it all is somewhat relative. If something is present in a couple states (see, e.g., Kennedy v. Louisiana) or for certain groups of people, it is not necessarily "usual." On the whole, it still would be "unusual," since in most cases, it is not a general rule. And, this is basically how proportionality review is now applied by the Supreme Court.*
Majority rule is a basic principle in this country and especially in the context of judicial review of punishments authorized by statutory law, "usual" is a reasonable way to limit discretion. It is still not the only way.
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* Prof. Colb talks about the issue more here, citing Justice Thomas regarding punishments that are allowed but rarely used. As she notes, arbitrary and discriminatory use can result as much as careful application to special cases. And, it is hard to remove something already passed, especially if it is rarely used anyways. Again, it's important to see the sum of the parts here.