According to Merriam-Webster, it's the word of the year. As defined by Stephen Colbert, it is “truth that comes from the gut, not books.”
“We’re at a point where what constitutes truth is a question on a lot of people’s minds, and truth has become up for grabs,” said the president of Merriam-Webster. “’Truthiness’ is a playful way for us to think about a very important issue.”
(ABC News, 8 December 2006)
http://abcnews.go.com/US/wireStory?id=2711832
