선무당이 사람 잡는다

Literal Meaning

An inexperienced shaman causes people pain.

Usage

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."

English Equivalent

A little knowledge is a dangerous thing.