YJLTG is an abbreviation for You Just Lost The Game. It originates from the /b/ image board on 4chan.org. On this image board there is a popular thread called The Game.
HOW TO PLAY:
You are now part of The Game.
You cannot win The Game.
The aim of The Game is to forget The Game.
Every time you remember The Game you lose.
Every time you lose The Game you must say outloud 'I lost The Game.'
By telling someone about The Game they are automatically part of The Game.
The Game never stops, once you are in you can not leave.
THE GAME - You just lost.
person one: let me tell you about The Game
person two: okay, what is the game?
person one: YJLTG