The spelling of the word "most airish" seems a bit confusing, but it actually follows the rules of English phonetics. In IPA transcription, the word is pronounced as /moʊst ˈɛərɪʃ/. The "air" sound in the middle of the word is represented by the "ai" letter combination, which is commonly used to indicate the long "a" sound. In addition, the "i" sound at the end is represented by the "ish" suffix, which can mean "having the qualities of." Overall, "most airish" means "most airy" or "having the qualities of air."
There is no specific word "most airish" in the English language, so it is not possible to provide you with the etymology of that particular term. However, if you have any other word or term for which you'd like to know the etymology, please feel free to ask.