What is Lost.?
A conversation ender. Used at the end of a statement when a person in the middle of a conversation wishes to stop talking about it immediately.
Coined from the TV show LOST, where at the end of every episode, the word "LOST" flashes across the screen, ending the episode very abruptly or on a cliffhanger.
Guy 1: "So I totally hooked up with that hot blonde last night at the party. I wish I had gotten her number."
Guy 2: "Dude, that was my cousin. LOST."