when in Rome, do as the Romans do

The advice goes back to a fourth-century question about fasting. The church at Rome fasted on Saturdays; the church at Milan did not. Augustine's mother was troubled about which rule to keep, and Augustine asked Ambrose, the bishop of Milan, what to do.

Ambrose's answer, as Augustine reports it in his letters, was practical rather than doctrinal: when I am at Rome I fast on Saturday, when I am here I do not — follow the custom of whatever church you are in, and you will give no offence and take none. The point was that the local practice was not worth a fight.

English shortened it to a rhyming couplet in the sixteenth century and dropped the theology entirely. When in Rome now covers everything from removing your shoes at the door to drinking whatever you are handed, and is very often said by someone talking themselves into it.