"The Hedgehog Song", more properly entitled "The Hedgehog Can Never Be Buggered At All", is a song made infamous by Pratchett's Discworld novels. We never hear the lyrics in canon, but the fans have made several versions.
Agent Stormsong insists he knows that the song is not true, but refuses to explain how he found out. Considering the excessively gruesome content of his home fic, nobody really dares to push the matter.