Loki's Wager, a form of logical fallacy, is the unreasonable insistence that a concept cannot be defined, and therefore cannot be discussed.
Loki is a Jötunn or Áss in Norse mythology, who, legend has it, once made a bet with some dwarves. It was agreed that the price, should Loki lose the wager, would be his head. Loki lost the bet, and in due time the dwarves came to collect the head which had become rightfully theirs. Loki had no problem with giving up his head, but he insisted they had absolutely no right to take any part of his neck. Everyone concerned discussed the matter; certain parts were obviously head, and certain parts were obviously neck, but neither side could agree exactly where the one ended and the other began. As a result, Loki keeps his head indefinitely (although in the specific example, he got his lips stitched shut as payback for getting out of the bet with tricky wordplay).
One may overcome the fallacy either by establishing a reasonable, working definition of the term in issue, or by showing that the other party is being unreasonable and avoiding the argument.