That word must be a slang for online speak to reduce the number of type hit. "깝대다" must be come from 의태어 "까불까불", "까불-대다" or "까불-거리다", and 경상도 dialect, "깝-치다" comes from "깝죽깝죽", "깝죽-대다" or "깝죽-거리다". "까불까불" and "깝죽깝죽" have almost the same meaning. The meaning of 의태어 "까불까불" is "위 아래로 가볍게 흔들며 움직이는 모양", and "깝죽깝죽" means "머리, 팔, 다리, 엉덩이등, 몸의 한 부분을 계속해서 흔들거나 움직이는 모양", Therefore, "까불-다" and "깝죽-이다" means "경솔(輕率)하다", "경망(輕妄)하다", or "건방지다", "잘난 체 하다"

may have something to do 깝치다, which is definitely slang. Original meaning is somewhat like "arrogant" or "let something get to one's head". But probably the native will be better to explain the subtlety. A korean girl once told me this is a word among guys and she professed that she did not even use it herself. But I am not sure it has something to do with 깝대다 at all. Just a guess.