Duh is a little sound people say when they think the last thing someone said — or asked — was obvious. So obvious that saying it seems silly. It means something like “well, of course” or “that was a slow question”. Young people use it a lot in speech and in messages. Between friends it can be playful; it can also sting, because you are telling the other person they have been slow. It is very informal. Do not use it with a teacher, a boss, or anyone you need to show respect to. Though actually, the sound was made to copy a heavy, slow way of talking, once used to laugh at people thought not to be clever — which is why even a joke duh can feel unkind.