The word "trustier" is spelled with the letter "u" in the middle instead of "o" because it is formed from the base word "trust" and the suffix "-ier", which means "more". The pronunciation is /ˈtrʌstɪər/, with the stress on the first syllable. The "u" in the middle is pronounced like the "u" in "hut", not "but". "Trustier" is used to describe something or someone that is more trustworthy than another.
The word "trustier" is formed by adding the suffix "-ier" to the word "trust". The suffix "-ier" is derived from the Old French "-ier" or "-er" and is used to form comparative adjectives. In this case, "trustier" means more trusty or reliable than something else. As for the word "trust", it comes from the Old Norse word "traust", which means confidence or security. It evolved through Middle English to become the word "trust" as we know it today.