4d

It is a korean slang word which means someone who is eccentric and wacky. It is not a word meant to insult a person, instead it is something which is used to complement him/her. This is most commonly used to describe Korean celebrities with a strong personality. It may also be used to indicate that a person has a very strong individuality or a unique personality. This is derived from ordinary people being "3d", so a person which is so quirky and thinks outside the box is considered "4d" or 4-dimensional.