The spelling of "most thickened" is straightforward when broken down phonetically. "Most" is pronounced as /moʊst/ with a long "o" sound and a voiced "s" in the middle. "Thickened" is pronounced as /θɪkənd/ with a "th" sound at the beginning, a short "i" sound, and a "d" sound at the end. The two words come together to form a phrase that describes something that has become thicker. Overall, the spelling is not difficult to understand with the help of phonetics.