The phrase "Tentacles or GTFO!" is derived from three phrases. "Tentacles" denotes that the user wants to see tentacles, but also implies that they want to see said tentacles inside something they don't fit in. A picture of Cthulhu isn't enough- Cthulhu has to be making a mortal suffer in unspeakable ways. "Or" implies a threat, while "GTFO" is the actual threat- failure to comply can result in you getting kicked, banned, or sodomized with tentacles (plural). The Internet is indeed serious business.