"What is a thrall?" said Túrin.
"A man who was a man but is treated as a beast," Sador answered. "Fed only to keep alive, kept alive only to toil, toiling only for fear of pain or death. And from these robbers he may get pain or death just for their sport. I hear that they pick some of the fleet-footed and hunt them with hounds. They have learned quicker from the Orcs than we learnt from the Fair Folk."
"Now I understand things better," said Túrin.
"It is a shame that you should have to understand such things so soon," said Sador



Tolkien, "Unfinished Tales"