Fruitcake is a cake made with candied or dried fruit, nuts, and spices, and optionally soaked in spirits. The color of it is brown, and in "Christmas Who?," it has a pink label that reads the food's name in green text.
Plankton makes fruitcake laced with Jerktonium as part of a plan to turn the Bikini Bottomites into jerks. He uses the Jerkmaker-9000 to dispense the cake to SpongeBob, only for the latter to prove immune to the Jerktonium's effects. Thinking his plan to be a failure, Plankton disgustedly leaves the Jerkmaker-9000 with SpongeBob as he goes to activate Plan B.
However, SpongeBob proceeds to distribute the fruitcake across Bikini Bottom while singing the song "Hot Fruitcake," inadvertently turning most of the population into jerks.