잔망꾼 is a noun describing a person who is eccentrically creative and playful. It's not a common word but seems to have become popular recently. It comes from 잔망하다/잔망스럽다 (adjective - playfully daring and resourceful, funny, etc) and -꾼 (suffix). The -꾼 suffix denotes a person with a certain characteristic, job or role, typically in the negative sense (but not necessarily in 잔망꾼). So 익살꾼 is someone humorous, 사기꾼 a swindler, and 방해꾼 someone who gets in the way of something, for example.