1)
Why are they called "Kesef Nim'as"?
1.
Rashi: It is not refined through any (even a great) effort.
2.
Radak: It is because they were not uprooted from their evil. Those who recognize their evil call them so, for Hashem despises them for not repenting.
3.
Sha'arei Teshuvah (1:23): One who recognizes his Creator knows how lowly and inferior is one who transgresses His words.
4.
Malbim: It is silver of impurities. Even though each sticks out its hooves 1 to justify itself, it is not to Hashem; Hashem despises them.
1
Bereishis Rabah 65:1 compares Esav to a pig, which sticks out its cloven hooves (one of the two Simanim of Tahor species), and feigns to be Tahor.
