A slingback is a woman’s shoe that is open at the back. Instead of a full cup around the heel, a thin strap — the “sling” — comes around the heel and holds the foot in. The front may look like a normal shoe or a sandal; the giveaway is that empty space behind the heel, with only a band. If the strap is too loose, the shoe slaps as you walk. You will hear the word in shops and in fashion writing. It names the shoe; it is not a verb.