When you receive a Wonderwall, a term used in Halo Reach
by me and my friends (and anyone else in the know, which is about to be you). A Wonderwall happens when you melee a person from behind, known commonly as a "back beat" or "beat back" depending on the person. The term Wonderwall is derived from the song by Oasis of the same name. "Back beat, the word is on the street that the fire in your heart is out". Since a Wonderwall is a one hit kill, it always put out the fire in your opponents heart.
Me- I just snuck up on someone and Wonderwalled them, now it's on to the next one