The prefix selector does check that there's at least ten words you can make with that prefix, at least one of which fills up the whole word. Your options were "zack", "zaire", "zakah", "zakat", "zamia", "zany", "zarf", "zayin", and maybe "zacat" due to some weird character encoding issue with "zacatón", I haven't actually tested it. And if you had six letters to use, you'd also have "zaddik", "zaffer", "zander", "zanier", "zanies", "zanily", "zapped", and "zareba".
No, I don't know what most of those words mean, either. I used an existing public-domain word list. "List every word in the English language" was well out-of-scope for a two-week jam.
RNG is just mean like that sometimes. There were several times during playtesting when it would come up with exactly one word I could play, and if that wasn't enough to kill, I was just screwed. Such is the nature of roguelikes, I guess.
Thanks for playing, glad you liked it!