The phrase "When in Rome" is an abbreviation of the longer expression "When in Rome, do as the Romans do." It means that when you are visiting a different place, you should adopt the customs and practices of that place. The origin of this saying is credited to Saint Ambrose, a bishop in the 4th century AD, who advised Saint Augustine to follow the customs of the church they were attending to avoid causing scandal. The phrase resurfaced in a more familiar form in 1777 in the Letters of Pope Clement XIV. Over time, it came to encourage travelers to embrace local culture and traditions, often justifying indulging in local customs and experiences.

Additionally, "When in Rome" is also the title of a 2010 American romantic comedy film starring Kristen Bell, where the protagonist experiences a whirlwind trip to Rome involving a magical fountain of love.

In summary, "When in Rome" as a phrase is an old proverb encouraging cultural adaptation, with historical roots dating back to the 4th century, and it is also a cultural reference used in modern media like movies.