Incidentally, I would explain the original meaning of "duh" as something like "это должно быть самоочевидно, и человеку с синдромом Дауна" -- although now it simply means "должно быть самоочевидно."

Almost without doubt, "duh" was first meant to imitate the speech of "retarded" people, or at least the cruel popular stereotyping of their speech. However, while that was the original meaning, nowadays people who say "duh" don't intend any offense towards those with real learning disabilities. (In a similar way, the word "moron" was originally a polite clinical term for persons with an IQ below 70; then it became a rude term for such people; today it means a person of normal intelligence who behaves stupidly.)

In the famous 1939 film version of John Steinbeck's Of Mice and Men, the simple-minded character Lennie (as played by Lon Chaney Jr.) does not say "Duh" even once, so far as I know. However, in the many, many parodies of the 1939 film (for example, from old Bugs Bunny cartoons), the parody version of "Lennie" is always saying "Duh! Duh, George, I wanna pet the rabbits, duh!" So, at least since the early 20th century, "duh" was already associated in the popular imagination with the speech of mentally-retarded people.