A world like onomatopoeia being onomatopoeic isn't that far-fetched. Perhaps the word could be named after a very common onomatopoeia, perhaps one that has gone out of use. A word with that sort of etymology is "shibboleth" - its current meaning is derived from what was a common shibboleth.
Well, my thinking was: Euroclydon's friend likes to be technically correct about the term onomatopoeia, and jimmyk chimes in with the same level of technical correctness on the matter. So for levity, I point out how it is 'ironic' in a nonsensical way - hoping to hit their OCD nerve a bit. As it is clearly not an ironic statement, and we only have anecdotal evidence of cucumbers being able to mutter a sound.
It would be very difficult to do that since onomatopoeia is a concept and concepts don't make sounds.