A Nathanist follows the doctrine of Nathanism. He/she can't tell the truth about anything and will always have a story better then your, even if its not true. See: Nathanism
Nathanist: "Oh you went snowboarding last weekend. I did that a couple o' years back on a 2x4, in the summer, in Argentina, met their president too, hes totally awesome and hooked me up with like 30 pounds of Oranges. Totally sweet."