When someone involuntarily falls asleep, 'passes the fuck out' or otherwise knocks themselves out and they end up unconscious on the ground they had just been standing on.
"Whoa! What's Timmy doing laying on the ground? "
"He's just taking a dirt nap. Sleeping off some of the booze, he's breathing, he'll be fine."
When you take a major fall off of typically an off road personal vehicle...ex: dirt bike, atv, quad, etc. This could also easily be applied to falling off of anything really...ex: bicycle, skateboard, rollerblades, stripper pole etc.
Holy sh*t dude! Did you see that douche bag try to jump over that dune on his quad? His leg is turned around backward! That was the worst dirt nap I've ever seen!