“Add” is an example of a word that’s 2/3 composed of one letter. “Lull” is 3/4 composed of one letter. But this is easy enough with short words (“I” is 100% one letter!). What words can you think of that are 6 letters long but are more than 50% one letter? 7 letters? 8 letters? Words with internal special characters (hyphens or apostrophes) are fine, and the special characters don’t count towards the word length.
I include some of my own answers below; post yours in the comments, if they yield results that are better than mine, and than the others you see in the comments. If you think someone’s answers (including mine) are incorrect, please check the usual sources before so asserting.