The phrase "most fallen in" is typically spelled as three separate words. "Most" is pronounced as /moʊst/ with a long 'o' sound and a soft 's' at the end. "Fallen" is pronounced as /ˈfɔːlən/ with a short 'a' sound and a silent 'e' at the end. And "in" is pronounced as /ɪn/ with a short 'i' and a single syllable. Together, the phrase means something or someone that has fallen in the most significant or extensive manner.