Used to express disdain for something deemed stupid or obvious, especially a self-evident remark.
This phrase was started with the blonde joke where a blonde calls the fire department because her home is on fire. When the person asked how to find her house she replied "Duh, the big red truck outside."
A woman walks out of her house to her car with her keys in her hand. A man calls out to her, "Going on a drive?"
To which she replies, "Duh, big red truck."