The act of incrementally moving, sliding or scooting one's derrière, buttocks or rear along when sitting on a sofa or bench, usually to make room for others to sit.
Probably derived from "scoot," which means "To move or slide to the side."
Brett, please schooch down a little so Aunt Dolores will have room to sit down.
You are playing Halo with the guys and you set up for a sniperkill, as you pull the trigger you say with a hint of coolness, "Scadoosh!" as they die. This can be followed by a victory dance or whatever you so please.