To emphasize that people should possess a sense of shame, decency, and moral integrity, just as even animals have their inherent nature or 'skin'. It's used to reproach those who act without propriety or self-respect.
他行事毫无底线，真是让人感叹相鼠有皮，人岂能不如鼠类？
He acts without any bottom line, truly making one lament that even a rat has skin; how can a human be inferior to a rodent?
Next Idiom