MPreg

The "what" of this definition is very simple: MPreg = male pregnancy, usually the result of bad slash. The "why" is something that many PPC agents still struggle to understand. Why do fanfiction writers feel that it will improve their stories to write a physically impossible occurrence between two men? Do they not have a basic understanding of biology? Unless that universe has sentient seahorses, MPreg is always absurd and is always a major pain for agents on a mission.

(There is one canonical instance of MPreg, in an episode of Star Trek: Enterprise, but it was entirely due to the natural biology of an alien species, and the unfortunate pregnant man was not actually the child's father, just a host for the alien embryo. Also, unlike most fanfiction MPregs, there was no slash relationship: the female alien accidentally impregnanted the human man simply by putting both their hands into a bowl of (what was essentially) pebbles at the same time. This does not mean that MPreg is canonical in Star Trek.)

The Cure
Sadly, fangirls who believe it's "cute" to have Jack Sparrow impregnated by Legolas will write what they will, and if there's nothing that logic and common sense can do to stop them, then our chances are slim to nil. Fortunately, the Department of Sufficiently Advanced Technology has come up with a remedy for such an event: the Embryo Extraction Kit. This resembles the de-bugger used in The Matrix, but hooks up to a life-supporting receptacle, depositing the embryo into a friendlier environment than some poor canonical's intestinal tract. Since the child of Jack Sparrow and Legolas could never exist in canon, the fetuses are under the care of Medical Department until birth, after which they are placed in Canon Nursery School.

The Division of Mpreg
The Department of Bad Slash's Division of Mpreg takes care of administering the above cure, and Bad Slash agents in general may often run across and have to fix Mpreg in their own fics.

See also Squick.