The phrase ‘When in Rome, Do as the Romans Do’ refers to the importance of adapting yourself to the customs of the people who are in a certain place or situation and behave like they do.
Example of Use: "Are you sure we should eat this with our hands?" Answer: "Why not? All of these people are eating it that way. When in Rome, do as the Romans do!"
This interesting idiom is often shortened to 'when in Rome' perhaps because it’s such a well-known phrase. The origin of the idiom 'When in Rome, do as the Romans do' was first seen in print in 1777, in Interesting Letters of Pope Clement XIV: “The siesta or afternoon’s nap of Italy, my most dear and reverend Father, would not have alarmed you so much, if you had recollected, that when were at Rome, we should do as the Romans do – cum Romanus eris.” This use suggests that the idiom was already well-known, and variations of it can be seen in examples dating from as early as the late 1500s.