豹変
A panther's change.
... 『易経（革卦）』の「君子豹変す、小人は面を革む（あらたむ）」に由来する。
これは、豹の毛は季節によって抜け替わり、斑紋が鮮やかになるように、徳のある君子は過ちを改めて善い方に移り変わるが、小人（徳のない人）は表面的に改めるだけで本質は変わらないといった意味である。
つまり、良い方向へ変化することを言ったものだが、現在では悪く変わる意味で用いられる。
豹変が悪い方に変化する意味となったのは、「小人は面を革む」までを含めたというよりも、「豹」という動物の恐ろしいイメージから連想させたものと思われる。
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.