깍두기 is a cube shaped side dish made from similar ingredients as those from 김치, but made from radish (무). This dish is much easier to prepare than other related kinds of 김치. It is especially common when eating certain soups.
깍두기 is a slang for a Korean gangster. The origin of the meaning is from the fact many Korean gangsters have a flat top like haircut and the shape is similar to the cube shape of the Korean side dish 깍두기.