A marvelously useful syllable that indicates that the statement immediately preceding is blatantly obvious, or that the preceding question has a blatantly obvious answer.
'Will Connecticut Republicans nominate for the Senate a former congressman with a reputation for bipartisanship, or a businesswoman whose claim to fame is building a professional wrestling empire? Duh."
Sound used to imitate a mentally retarded person as an answer to an obvious remark or question. The person using "duh" is thereby sarcastically indicating that he/she is not retarded and the forgoing remark or question is obvious to him/her.
Him: Hey, to insert a column you have to press the column insert menu!
Me: Duh! No kidding!