Provided you speak greek, how would you respond if someone used the word “ταχυδρόμος” for someone crossing a distance fast, not for the postman?

To explain the question:

Tachy-dromos, “quick-runner”, was originally a word for a courier. Couriers deliver mail, and tachy-dromos is now the word for mailman.

If someone uses tachy-dromos in its original original meaning of “fast runner”, rather than its community accepted meaning of “mailman”, how do I react?

I react by telling them to stop being Humpty Dumpty. Words don’t mean whatever you decide them to mean, and they don’t mean whatever the original etymology of the word says, even if the etymology is transparent. Just as an undertaker in English no longer means “entrepreneur”, someone who takes on an “under-taking”. Words mean what the community has ended up saying they mean.

There’s an out with that kind of compound: you can pause between the compound words: “He’s a quick. runner. Not a quickrunner.” Like in English, if I were to say “an underTAKE. -er. Not an undertaker.” That would try and trigger the etymological reading in the listener.

And would likely fail.