Impse

Impse is the "act of copulating with an imp". Impse first appeared in a piece of fan fiction posted in the Doomworld Forums and was popularized further by the joke WAD Imp Encounter. The name is a play on goatse, a famous shock site.

The original story
As many noted, the Encounter story is written in such a manner that the absurd nature of the subject matter takes a back seat, in many ways, to the author's commentary on the degradation of mental capacity that occurs to soldiers exposed to prolonged periods of combat and warfare, much as veterans of Vietnam have. What was meant to be an satirical joke about the overindulgences into seriousness that fan fiction writers often employ actually became the perfect piece of fan fiction for Doom, as this psychological deterioration is emphasized during the game's sparse plot-related text. Even Doom's original slogan, "Where the sanest place is behind a trigger," seems to suggest that the Imp Encounter's idea about the hero of Doom suffering from severe isolation and traumatic stress problems might be valid.

Appearances
Impse makes an appearance in many Doom WADs, including Action DOOM, Massmouth 2 and the secrets in Deus Vult. On ZDaemon, it experienced some fame as a secret appearing in a small number of ZDCTFMP2 maps, and became best known through graffiti in the MAGHQ WAD, where the first frame of the animation is captioned "Mike loves imps."