The high-five is one of the best greetings, with kids in particular. People who aren’t huggers or hand-shakers often prefer a high-five. The gesture is usually prefaced by actually saying “high-five” out loud, so that the other person knows what to expect. “Don’t leave me hanging” is said in reference to someone who doesn’t complete the high-five act - and leaves your hand awkwardly hanging in mid-air. It has now become a metaphor for letting someone down in some way.