noun. A less genetically dubious--yet still unadvisable--form of incest.
Real citation: "Are we talking about roommate-cest here? Bad idea buddy, really bad idea."
(zesovietrussian, Dec. 14, 2005, http://www.geekculture.com/ultimatebb/ultimatebb.php?ubb=get_topic;f=7;t=000128)
Made-up citation: "Roommate-cest won't produce any web-footed infants, but paying the cable bill may become an issue when the romance fades."