Often I can guess where welsh words came from but this one doesn't remind me of anything in English, French/Latin etc. I wonder where the word coban came from? Obviously not the same as English but is this another Latin root or an even older one? Or something else?
The 'Geiriadur prifysgol cymru - University of Wales dictionary' has very little information about it except to refer to the first entry of this word as being in 1794.
Here's something that makes sense, the word for 'cope' which is vestment in RC church is 'cob' in Welsh, 'cob' is also an old word for 'cloak, robe and coat'
Coban in an older dictionary is a woman's coat, so it must be derived from 'Cob', which was probably used for a man's coat at some time.
In modern times the word 'côt' has become used for coat, thus leaving 'coban' to refer to women's nightwear only.
GPC also shows -an as a diminutive suffix, so perhaps cob + -an -> coban might give us 'a small cloak/robe/coat' -> a night-gown?