One who has a cork up their ass, either literally or figuratively. A literal corkass should seek medical attention to remove the cork from their ass. A figurative corkass constantly acts uptight and agitated, as if they do, in fact, have a cork lodged in their asshole.
"Doctor, there's another corkass in the surgery ward that requires attention."
"I asked my friend for a lift to work today, but he said no on account of it being out of the way. What a corkass."
One whose ass is so full of blackheads, acne, hair that was shaved about 2 days ago so that it's just now growing back, or any sick combination of the 3, that it has the appearence of cork.
kid 1: "Okay, I officially saw the grossest thing in the locker room today."
kid 2: "wtf do i want to know?"
kid 1: "probably not but i'm gonna tell you anyway. that kid etherage is officially a corkass."
kid 2: *pueks*