noun. An eight-legged ass.
Real citation: "Behind the targeted attacks is the crude Karnelli, an infamous underworld mogul.
Spidey realizes his old nemesis, Shocker, is behind the quake that buried them underground, and manages to stop the vibrating-villian…for now. However, one of the passengers reveals his identity to Spider-man, exposing his relationship to a particular pain in Spidey’s arachno-butt."
(Nov. 20, 2008, Spider-man Crawl Space, http://www.spidermancrawlspace.com/wordpress/?p=1141)
Made-up citation: "I wrote my dissertation on diseases that afflict the arachno-butt, and what they mean for today's Generation Z spider. But the job market is awful... GE just laid off eight arachno-proctologists Wednesday."