"The fourth wall" is an expression stemming from the world of theater. In most modern theater design, a room will consist of three physical walls, as well as a an imaginary fourth that serves to separate the world of the characters from that of the audience.
In fiction, "breaking the fourth wall" often means having a character become aware of their fictional nature. This can range from your character advising you to "Press X" in a tutorial all the way to Psycho Mantis reading your memory card and mentioning the other games you've been playing. However, the most direct violation of the fourth wall would be a character openly acknowledging they are in a video game or even directly speaking to you, the player, instead of to your character.
(Jak 3:)
Monk: This isn't a game!
-Jak and Daxter look toward the player with confused expressions-
When someone makes a comeback so epic, it gives all Canadians orgasms, if you know what I mean. Furthermore, the wave of awesome coming from the comeback itself is so powerful it leaves the offended and the audience speechless, making grounds for the offender to yell the word. Originated when Agent FD killed all the zombies of LA by using slips of the tongue.
Fourth Drink: "Your face!"
Person B: "..."
Fourth Drink: "I fourthed you like maple syrup on spaghetti!"
An internet meme that orignated from sega's "Shadow The Hedgehog." In a level cutscene shadow is looking for the 4th chaos emerald and he see's Tails who has the damn 4th chaos emerald.