Why is the "perjurious oath" (al-yamin al-ghamusa) so named? Is it because its taker is plunged into the Fire or plunged into sin? And what is its expiation?

It has been authentically reported from the Prophet, peace and blessings be upon him, that he counted the yamin ghamus (perjurious oath) among the major sins, because it immerses its taker in sin and then in the Fire. The majority of scholars hold that there is no expiation (kaffarah) for it, and what is obligatory is repentance to Allah and returning rights. However, Imam Al-Shafi'i and others held that expiation is obligatory. Combining repentance and expiation is more cautious.