Pathetic loser who leaves his friends in the lurch when they needed him most. This saying may have been spawned in 2005 by the children's cartoon, "Foster's Home for Imaginary Friends".
Character 2 is attacked by a monster and his friend bugs out instead of staying to help him. After #2 gets stomped, Character 1 walks up and asks:
Character 1: "Are you ok??"
Character 2: "Sure. (pointing at the unloyal friend) No thanks to Ditchy McAbandonpants here."