The correct spelling of the phrase "most preventing" is /məʊst prɪˈvɛntɪŋ/. The word "most" is spelled as usual and the word "preventing" is spelled with a double "n" after the prefix "pre-". This is because the stress falls on the second syllable of "preventing", and when a prefix ending in a vowel is added to a base word that also ends in a vowel and the stress falls on the second syllable, a double consonant is usually added to the base word.
"Most preventing" does not have a specific etymology because it is not a standalone word or phrase. "Most" is an intensifying adverb meaning "very" or "extremely". On the other hand, "preventing" is the present participle of the verb "prevent", which comes from the Latin word "praeventus" meaning "to come before, anticipate, hinder". However, together, "most preventing" is not a common or standard phrase in English, and it is unclear what specific meaning or context it would have.