Once a (something), always a (something) proverb
A person cannot change his or her innate or fundamental nature.
He said he would quit alcohol, but he still can't desist from drinking. Once an alcoholic, always an alcoholic.
People lost their faith in this celebrity as they think that once a liar, always a liar.
Despite his old age, he still managed to beat the burglar breaking into his house last night as he used to be a phenomenal boxer. Once a boxer, always a boxer.
The origin of this proverb is not clear.
People tend to like forbidden or illegal things just because they are forbidden or illegal.
He must have believed that stolen fruit is the sweetest, and he, therefore, cheated on his partner.