Soon Bob heard from the hillside the "hy-yi-hi, whomp, whomp, whomp!"
Pete Whitney said, "You git a smart lot of work out'en a farmed-out young'un if you whomp him to it."
1926, echoic of the sound of a heavy blow or something falling heavily.
1952, from whomp (n.). Related: Whomped; whomping.
[echoic fr the sound of a blow, perhaps influenced by dialect whup and whop, ''whip'']