A combination of "heinous" and "gnarly". Used to describe something that is both very difficult, painful, or dangerous (heinous) and sick, cool, awe-inspiring (gnarly). Most common usage in the extreme sports world.
"Travis Pastrana's double backflip in the X-games was heingnar"
"I cant believe that you just threw down a 360 air over that pit of lava while being shot at with pellet guns; that was so heingnar!"