About

"The Cake is A spy" is a spin-off about The Cake is a Lie, which is being a very popular fad.
The phrase is parodying Spy's ability to turn into another character in Team Fortress 2, a popular video game made by a video game company Valve for the PC and XBOX360. Valve also made video games such as Half-life and Portal.

Orgin

"The Cake is A spy"'s orgin is probably by the user Strangemodul3 in the website FPSBanana,
being as a spray in the game that you can install. A year later a user named S_coholic made "The cake is a spy" spray too, this time for Gamesprays.