A way to tell someone that something's wrong, even if they don't know about it. Taken from season 5, episode 2 of Gilmore Girls.
*Lorelai approaches Miss Patty's dance studio where Patty is instructing several small dancing apples.*
PATTY: Keep it going, kids. You're red, you're delicious. You're about to have the juice sucked out of you. *She smiles as Lorelai passes* There she is -- our spunky entrepreneur.
LORELAI: I am the uber-Trump-Murdoch-Maximus. *She glances back* Oop. Apple down.
PATTY: Apple down. *pauses before understanding then reacting* Apple down!