They were referring to a temporary slave whose labor would be auctioned off to a member of the public, with the money going to the state. Prisons, in our present sense, were rare, although they were coming in.
My point, poorly worded though it was, is that the convict labor approach wouldn't work in today's economy, for a host of reasons. Among others, you'd have to pay somebody more to watch him than he would generate in labor.
I seriously doubt that the authors of the Amendment were referring to a criminal sentence of hard labor as served in a prison.
I certainly could be argued, just as the meaning of "cruel and unusual" is. Tangentially, I always wondered how the military draft squared with that clause. Enrollment in an army against your will and being forced to labor and risk life seems to be against this amendment.