[MMM]

It is a term of endearment of implying closeness. The older you get the less often it is used, but it sometimes appears in nicknames that can stick around for a lifetime.

Because of the intimate nature, it is rarely used in offices or other professional, business-like situations.

Some of my elder teachers I worked closely with in Japan called me "sensei" at school, but used my name and "~chan" after a couple beers. These were teachers I had worked at least a year with, and were old enough to be my parents, which is probably why "~chan" came out after they loosened up.