It is the ultimate dare. It's for when a person rejects a regulardare, AND a double dog dare. It is a dare that you HAVE to do under any circumstances. If you don't do it you look like a weakling or a chicken.
I triple dog dared her to run around the house completely naked. She chickened out and to this day we don't let her live it down.
Me: "I want you to go clean out my toilet after that massive shit I justtook."
person: "No, I'm not doing that."
Me: "I triple dog dare you."
person: "Deal!"
Note: The infamous Triple-dog-dare is infamous for the sole reason that you cannot back down from it--without exception! An issued triple-dog-dare has no counteraction and must be implemented and/or carried out.