A word used when a person makes a topic that tries to be funny/tries to make it to 500. When the topic would fail people would yell out "FISSION MAILED!" Instead of "MISSION FAILED!"
The origin of this phrase may come from Metal Gear Solid 2. At one point late in the game, a false version of the Game Over screen (which read MISSION FAILED) came up which read FISSION MAILED.