선무당이 사람 잡는다
An inexperienced shaman causes people pain.
Used to comment on or to warn people about a person who is pretending to know more than he or she really does.
무당 means "shaman," and "선" means "inexperienced," so 선무당 means an "inexperienced (novice) shaman." 사람을 잡는다 means "to give people pain by using words, labor, or a whip."
A little knowledge is a dangerous thing.