When you place your wang between the butt-cheeks of your bedmate. Sometimes an innocent byproduct of spooning, sometimes a not-so-subtle hint that you're in the mood, occasionally purely for a laugh.
What a potential date does when they want to keep you available, but don't want to commit to a specific date. They may ask your availability, but not commit to a date per se. You are on the back burner, so to speak.
When a friend leaves you with a person they just met in order to pursue another person they find more attractive. You are suppossed to keep that person occupied in case they stike out with the second.