So I've got a cold, I've stayed up all night sneezing and sipping my decaf green tea in search for the correct terminology of this garment (pictured below). Without any help from the Fashionista community, I have thus far come up with the following:
I'll update tomorrow with more images of this mysterious item. But most likely the name will be completely obvious, I'll look the fool and as a result of my ignorance I will recoil in defeat and profusely apologise for wasting your time. Until then... happy hunting.