One who, in an argument, takes a position, and when that position is destroyed by evidence and well-reasoned argument, acts as if the reasoned argument never happened, instead sticking to his original position while resorting to logical fallacies such as bad math, unwarranted assumptions, appeals to emotion, wishful thinking, false and deliberately misleading data and analysis, or ad-hominem attacks on his opponents. One who argues in bad faith.
Paul Ryan is a derp.
Salim Furth is derpy. Can put links here, search on yes-europe-really-is-in-the-throes-of-austerity
Avik Roy exhibits derpitude. Search New Republic. Search also Krugman blog bad-faith-and-civility-health-care-edition: "If someone says the sky is green, you prove that it’s actually blue, and the next day he comes back once again insisting that the sky is green, and this happens repeatedly, you eventually have to acknowledge that mannerly debate about the color of the sky just isn’t enough; you have to go meta, and talk about the fact that this guy and his friends just aren’t in the business of honest discussion."