it's a dress you wear for "cocktails" which was a big social event in the 40's and 50's...you'd go out for "cocktails"....

I dunno why they call them cocktails...

cock... tail... (she's wearing a cock.. tail) it's a little funny... I looked up the origin...

masonic meetings, guy mixed drinks in a egg cup and served them, name of the egg cups were "cocktay" that's how the name came about