Backless describes clothes — a dress, a top, a gown, a swimsuit — that leave a woman’s back open. The cloth does not cover the back, or covers only a strip or a little of it, so you see skin from the shoulders down. It is about the cut of the garment, not about a person having no back. You almost always hear it stuck to a clothing word: a backless dress, a backless swimsuit. It is ordinary fashion talk. It is not a rude word by itself; how much back is shown, and whether that fits the event, is a separate, social question. You will not normally use it for chairs or buildings — those have other words.