Once a —, always a — proverbial saying, early 17th century, meaning that a particular way of life produces traits that cannot be eradicated.
once a priest, always a priest proverbial saying, mid 19th century, a specific application of the preceding proverb. An act enabling the clergy of the Church of England to unfrock themselves (the Clerical Disabilities Act) was passed on 9 August 1870.
once a whore, always a whore proverbial saying, early 17th century.
once bitten, twice shy proverbial saying, mid 19th century, meaning that someone who has suffered an injury will in the future be very cautious of the cause. The variant once burned, twice shy is found in the US, and may have been influenced by a burnt child dreads the fire.
See also once in a blue moon, fool me once, shame on you.