Also known as: 下着エプロン, 水着エプロン
An Apron worn with No Shirt underneath. It may be worn with Underwear, a Swimsuit, Pants, or other such clothing, but not with a top beneath the apron.
If the apron is only worn with items that don't cover the private parts, such as Thighhighs, Elbow Gloves, Hats, etc, then it's a Naked Apron instead.