語源は不明だが、おそらくオノマトペ、大きな物が地面にぶつかる音から。bump、clump、dump、pumpと比較。
LUMPING. Great. A lumping pennyworth; a great qualtity for the money, a bargain. He has got a lumping pennyworth; frequently said of a man who marries a fat woman. [Grose, "Classical Dictionary of the Vulgar Tongue," 3rd edition, 1796]