About
X is a Spy is famous quote from popular first-person shooter Team Fortress 2. Ever since the game's release, there has been various variations and parodies of the quote all over internet, usually at different kind of discussion boards and image macros.
Orgin
In Team Fortress 2, the Spy class has an ability to disguise themselves as members of enemy team in order to backstab players, destroy Engineers buildings, and generally sabotage their efforts. If the enemy suspects one of his teammates being Spy, he can alert his friends by looking at the dubious player and using x+2 default key. Depending on the class you're playing and the Spy is disguised as, your character uses some variation of "X is a Spy!"