Definition
pure chance; luck
How People Actually Use It
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!!"
Source: Urban Dictionary