Curiousity question: Did Joseph or any other church leader ever say what the words “Mormon” or “Cumorah” mean?
I know of no writing that gives the etymology or defines either the word Mormon or Cumorah. Although proper nouns generally derive from some source, there is nothing known about the language of the Book of Mormon people other than that which is written in the Book of Mormon itself. However, I can give you a reference that authoritatively declares what the word Mormon does not mean. These are the words of Joseph Smith, the Mormon prophet, who, by the gift and power of God, translated the Book of Mormon from its original Egyptian text into English.
Sir:–Through the medium of your paper, I wish to correct an error among men that profess to be learned, liberal and wise; and I do it the more cheerfully, because I hope sober-thinking and sound-reasoning people will sooner listen to the voice of truth, than be led astray by the vain pretensions of the self-wise. The error I speak of, is the definition of the word “Mormon.” It has been stated that this word was derived from the Greek word mormo. This is not the case. There was no Greek or Latin upon the plates from which I, through the grace of God, translated the Book of Mormon. Let the language of that book speak for itself. On the 523d page, of the fourth edition, it reads: “And now behold we have written this record according to our knowledge in the characters, which are called among us the Reformed Egyptian, being handed down and altered by us, according to our manner of speech; and if our plates had been sufficiently large, we should have written in Hebrew: but the Hebrew hath been altered by us, also; and if we could have written in Hebrew, behold ye would have had no imperfection in our record, but the Lord knoweth the things which we have written, and also, that none other people knoweth our language; therefore he hath prepared means for the interpretation thereof.” (Times and Seasons 4:194)