Listing this pattern, Simplicity 3998, into my shop, the title jumped right out at me .....
Blouse-Slip? I'd not heard that term before. So, of course, headed right over to Google, and found ... nothing. I did notice that Wikia Vintage Patterns have a Blouse-Slips Category, althought there is nothing in it.
I can see where it would come from. The top is like a blouse and the bottom does resemble a half slip, minus the darts, of course. Cute collar variations .. would you say that is a round collar?
I'd say semi-fitted slim skirt. A very nice mad men style, authentic vintage early 1960s.
This pattern is available in my shop should you like to stitch up your own. Imagine, at lunch, with your friends .... "How to you like my new blouse-slip?" There are, of course, literally hundreds of other dresses as well.