That's a new one for me. You do sometimes see Latin phrases used in place of their more commonly known English versions. It looks like the closest common translation is something like "in the midst of things", or "starting in the middle".
These Latin (french, etc) phrases are more often than not used to either:
- Emphasise a very specific meaning that doesn't translate well.
- Try and look clever.
- Hold a specific legal definition.
- Use a commonly known foreign saying (laissez faire, or et al, for example)
They are not used often in conversation, more often you'll find them in journalistic writing.
In this case it seems this is, quite specifically, used as a stage direction in plays, outlining the scene.
You quite often get Latin and Italian loan words and phrases in plays and music. Pianissimo, for example.