Once a teacher, always a teacher.

한번 선생님은 영원한 선생님

요새 제가 읽고 있는 책(The Lightning Thief)에서 아래와 같은 대화가 나옵니다.

"By who?" 

"By whom," Chiron corrected.  Once a teacher, always a teacher. 

"By you."

어..이런 비슷한 표현 어디서 봤더라..해서 찾아봤습니다.

Once a something, always a something.

  used to say that people stay the same and cannot change the way they behave and think 출처

  A person's innate or fundamental nature is not something they are willing or able to change.출처 

  A person cannot change their fundamental nature. 출처

예문

Once a flirt, always a flirt.

Once a thief, always a thief.