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Phrase(s): Once a priest, always a priest. [and] Once a whore, always a whore.
Prov. A person who has done a certain kind of job will always have the characteristics of people who do that job, even after he or she no longer does that kind of work. (This can be applied to many different occupations.) • Alan: My cousin left the clergy, but boy! He still preaches at me all the time. Jane: Once a priest, always a priest, huh?
McGrawhill's American Idioms And Phrasal Verbs