Katy Waldman delves into it:

Truthiness is “truth that comes from the gut, not books,” Colbert said in 2005. … Scientists who study the phenomenon now also use the term. It humorously captures how, as cognitive psychologist Eryn Newman put it, “smart, sophisticated people” can go awry on questions of fact. Newman, who works out of the University of California – Irvine, recently uncovered an unsettling precondition for truthiness: The less effort it takes to process a factual claim, the more accurate it seems. When we fluidly and frictionlessly absorb a piece of information, one that perhaps snaps neatly onto our existing belief structures, we are filled with a sense of comfort, familiarity, and trust. The information strikes us as credible, and we are more likely to affirm it – whether or not we should.