A lyth is a mixture of a lie and a myth. It is used to degrade or to afflict by posing as if something that cannot be proved has imposed the degradation. For instance, the idea that slaves had no souls was used to denigrate slave. One can not prove nor disprove the existence of souls. The lyth was used to appease the conscience of the slaveholders and further harm the slaves.
The slaves endured the lyth that they had no souls.