What is this and when is it from?

[plousia]

I'm guessing nightgown? I did wonder if it could be some kind of ethnic robe. It has this very strange pocket on the side just under the right sleeve. The sleeve gussets appear to be a later addition, of a different sort of fabric, "crisper" than the main fabric.

Any info on this appreciated, especially dating, what the heck that pocket is for, and what the embroidery is called?