(some people) are on a first-name basis

When people in English-speaking countries get to know each other well, they often start to call each other by each other's first names. People who use each other's first names are "on a first name basis".

If someone calls a person like a teacher, a political figure, a celebrity, etc. by their first name, you can tease this person:

Oh, are you on a first name basis now?

This phrase appears in these lessons: