no obj., with adverbial
1- make such a sound发出低沉的声音, 发出砰声：
he pitched a snowball that whumped into the car.
他掷了个雪球, 砰的一声打在汽车上。
1.1
- with obj. strike (something) heavily with such a sound重击, 猛击：
she began whumping him on his lower back.
她开始猛捶他的后背下部。
词源
late 19th cent.: imitative.