"a heavy blow," 1923, of echoic origin.
(also whammo) An exclamation signaling the suddenness, violence, surprise, etc, of a quick, sharp blow: Wham! suddenly the meaning hit me/ And then—whammo—she was blindsided (1932+)
To hit; strike; sock: And the whamming continues
[1925+; echoic, and related in sound symbolism to whip, whale, whack, whomp, whop, and other wh-words denoting blows]