The single person in a group who rejects a joke as unfunny, while all others regard it as devastatingly funny and potentially capable of causing 6 million people to die of laughter.
Often someone who is generally considered to have a poor or weak sense of humour.
Phil: Dave didn't laugh once during that Flight of the Conchords show
Harry: I know man, what a lolocaust denier
A person who denies the hilarity of a given joke; one who bums the joy.
Larry was such a lolocaust denier. He didn't even crack a smile at that joke!