This word originates from the I Ching (The Book of Changes), one of the five ancient Chinese classics, and comes from the particular phrase: "The man of wisdom changes his spots like a panther, while the small man only hardens his face into leather."
The meaning is derived from the natural behaviour of panthers, who renew their spots during their seasonal shedding of coats: i.e. The man of wisdom and virtue will correct his mistakes, learn from them and better himself; however, the little man will change, but only superficially, while his core self remains the same.
This saying is used to describe a change for the better, but in present times has acquired pejorative connotations.
While this saying includes the original nuance ("the small man will only harden his face into leather"), its negative meaning is thought to have come from the association of the Chinese character for panther (豹) with the fearsome image possessed by its animal namesake.