Yameen ghamoos refers to a false and evil oath, such as that in which a person unlawfully consumes the property of another. It is called ghamoos because it immerses (taghmis) the one who does that in sin and then in Hell. This was stated by Ibn al-Atheer in al-Nihaayah (3/724).
It says in al-Mawsoo’ah al-Fiqhiyyah (35/41):
The fuqaha’ differed concerning the necessity of expiation in the case of a false oath. There are two points of view:
1 – That it is not obligatory to offer expiation for a false oath. This is the view of the majority of fuqaha’: the Hanafis, Maalikis and Hanbalis.
2 – That it is obligatory to offer expiation for a false oath. This is the view of the Shaafa’is.
Each group offered evidence to support its view. End quote.
See: Badaa’i’ al-Sanaa’i’ (3/3), al-Taaj wa’l-Ikleel (3/266) and Kashf al-Qinaa’ (6/235).
It says in Fataawa al-Lajnah al-Daa’imah (23/133):
Yameen ghamoos is a major sin, for which no expiation is sufficient because it is such a grave sin. No expiation is required for it according to the more correct of the two scholarly opinions; rather what is required is repentance and prayers for forgiveness. End quote.
Whether one says that expiation is required or not, the expiation does not expiate the sin of a yameen ghamoos. Rather it is essential to repent sincerely.
Hence Shaykh al-Islam Ibn Taymiyah said in Majmoo’ al-Fataawa (34/139), after mentioning the difference of scholarly opinion concerning expiation for a false oath:
But they are agreed that the sin is not erased merely by offering expiation. End quote.
A false oath, like other sins, may be expiated by sincere repentance. There is no sin for which repentance cannot be accepted, for Allaah has opened the door of repentance to every sinner, and Allaah accepts the repentance of the one who repents.
Allaah says (interpretation of the meaning):
“Say: O ‘Ibaadi (My slaves) who have transgressed against themselves (by committing evil deeds and sins)! Despair not of the Mercy of Allaah, verily, Allaah forgives all sins. Truly, He is Oft‑Forgiving, Most Merciful”
Ibn Katheer (may Allaah have mercy on him) said:
This verse is a call to all sinners, kaafirs and others, to repent and turn to Allaah. It tells us that Allaah forgives all sins for the one who repents from them and turns away from them, no matter what they are, no matter how many they are, even if they are like the foam of the sea. There are many verses on this topic. End quote.
See also the answer to question no. 46683 for more information.
And Allaah knows best.