A response, when something you've seen or you've heard is beyond description. It could be good, bad or indifferent.
The late Royal Marshal coined this phrase and all credit goes straight to him.
You college rivals football team scores a td on a last second Hail Mary to win the game....Just Damn.
You're in Vegas and you hit the one arm bandit for $5 million. Just Damn.
You get pulled over by the cops doing 80 in a 65. Just Damn.