괜히 = 공연히 : 아무 까닭이나 실속이 없게 It means "unnecessarily, needlessly, uselessly,fruitlessly, pointlessly,in vain, for nothing, to no purpose" ex) 그걸 괜히 샀어요. I regret buying it. 괜히 울고 싶은 기분이에요. Somehow I feel like crying.