Unexpected good luck.
A fine ass naked broad fell out of the sky onto my lap. That would be Dumb Luck!
When dumb person achieves what he doesn't deserve or when a deserving person fails to achieve,luck is called dumb luck
Norah:Hey, Sam should have been student of the year for his outstanding performance throughout the year.
Adam: Dumb luck!!!
Norah:How come hurt locker won Oscars?
Adam: Dumb luck!!