Ben Zimmer in The New York Times:
Around 4 p.m. on Oct. 17, 2005, Stephen Colbert was searching for a word. Not just any word, but one that would fit the blowhard persona that he was presenting that night on the premiere episode of Comedy Central’s “Colbert Report.” He once described his faux-pundit character as a “well-intentioned, poorly informed, high-status idiot,” and the word he was looking for had to be sublimely idiotic. During the rehearsal, Colbert was stuck on what term to feature for the inaugural segment of “The Word,” a spoof of Bill O’Reilly’s “Talking Points.” Originally, he and the writers selected the word truth, as distinguished from those pesky facts. But as Colbert told me in a recent interview (refreshingly, he spoke to me as the real Colbert and not his alter ego), truth just wasn’t “dumb enough.” “I wanted a silly word that would feel wrong in your mouth,” he said.
What he was driving at wasn’t truth anyway, but a mere approximation of it — something truthish or truthy, unburdened by the factual. And so, in a flash of inspiration, truthiness was born. In that night’s broadcast, he imagined the disdain his coinage would engender among elitist dictionary types. “Now I’m sure some of the Word Police, the wordinistas over at Webster’s, are gonna say, ‘Hey, that’s not a word,’ ” he said. As I pointed out at the time on the linguistics blog Language Log, truthiness already appeared in the Oxford English Dictionary under the adjective truthy. To be sure, it was exceedingly rare before 2005, but it had been recorded as a somewhat playful variant of truthfulness since the early 19th century.
Regardless of its pre-Colbert history, truthiness in its satirical new meaning charmed many a wordinista.