Informal Used to emphasize the suddenness of an event.
So, I'm driving along when, whammo, a tree falls in front of the car.
"Sure, the boys say, 'What's the use? I'd just get started and whammo, I'm gone.'"1951 November 5, “The Younger Generation”, in Time Magazine:
2 more examples
Every year I try to catch the first buds as they break out of their winter closets, and every year I miss. One day you hit the street on your way to your day's routine and there it is, whammo, LIFE, popping up and out all over the place.1983 April 16, Nancy Walker, “Spring Surprises”, in Gay Community News, page 6:
He lifted one foot and set it down again, whammo, but Ed was so engrossed in Pynchon's novel that all he recalls is tromping the scorpion to death with his stung foot, then quickly fetching a bucket of ice water, jamming the foot into it, and continuing to read.1988, David Quammen, The Flight of the Iguana:
[…] dawdles for 15 minutes before presenting a mild whammo. The whammos start at about 33 minutes and then pile up at 36 minutes, 40 minutes, 50 minutes, 59 minutes, and 64 minutes.2006, David Bordwell, The Way Hollywood Tells it: Story and Style in Modern Movies, page 113: