It means "dead right there" when speaking about self defense shootings.
It usually refers to when the target drops, and instantly expires. Another way of saying it is "dead on their feet" or "dead man walking" to mean someone mortally wounded who is still ambulatory, if only momentarily.
It became a saying sort-of in opposition to the usual "DOA" mostly by the police when a person was obviously dead with no need to rush to the hospital.
Since it's been answered and isn't really a handgun topic, we'll call this thread DRH.