Definition of "wham"

I was off duty, minding my own business, […] when I heard a gun go off. Wham! it roared, not more than twenty feet from me, and then, in quick succession, wham—wham!

1950 July 22, Ferguson Findley, “Waterfront”, in Louis Ruppel, editor, Collier's: Incorporating Features of the American Magazine, Crowell-Collier Publishing Company, page 16

Wham! Overnight he [Dwight D. Eisenhower] became a warmonger.

1952 September 22, “Since Stevenson Prefers 'Compromise', Foreign Policy Is Squarely in the Campaign”, in LIFE, volume 33, number 12, Time Inc., page 30

I'll look the exact same way for a decade, and then— wham!— God hits the progeria switch and for two years the downhill plunge begins anew.

2008 August, Douglas Coupland, “40 on the Outside, 30 on the Inside: My theory of how men really age”, in Best Life, volume 5, number 6, Rodale, Inc., page 77