Sucking the joy out a given area of interest, effectively damaging or destroying any related hopes and dreams.
It is named for the feeling you get when, as a child, you are told that Santa doesn't exist. Thus, killing Santa.
Guy 1: "You know, Taco Bell's meat isn't REALLY 80% beef"
Guy 2: "Way to kill Santa, guy"
Guy 1: "You know no girl is ever going to like you"
Guy 2: "You know you're killing Santa for me, right?"