We have found one of the words used only once in The Hobbit!

• 1.066 Dump the crocks in a boiling bowl;

This calls for an OED moment, and we are not disappointed:

But the sense of the word has evidently received onomatopoeic modification, from its suggestiveness of a dull abruptly-checked blow or thud, and of the action producing this: compare thump

“dump, v.1.” OED Online, Oxford University Press, June 2019, http://www.oed.com/view/Entry/58418. Accessed 3 September 2019.

Yes, indeed, we can add this to the sound-play words! I might add that the noun meanings of “dump” include a kind of music that makes you feel down in the dumps.