Origins of em?

First of all, sorry for my bad English; I’m not a native speaker.

I look for historical information about origins of the term em (typographic unit of measurement).

Many sources state that it descends from the letter M, since sometime and somewhere it had exactly square dimensions. This sounds credible, but I fail to find any reliable historical evidences.

For instance, when and where was the term em used for the first time? Who has invented it?