A sexual act involving three willing partners, two of whom must be male. One of the chaps doffs his trousers, while the second penetrates him from behind - both are standing up. The third member of this unholy trinity then performs oral sex on the man who is being penetrated. The origin of the term is unknown, but there may be a tenuous Rasputin link.
Percy had never been much of a one for threesomes, but at least in a Russian roasting he got to see the end of Will & Grace.